diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2a357ade9..1951fcf2e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -41,7 +41,8 @@
 - Add logger information on handling of stopIteration error (#960)
 - Replace deprecated ConfigSpace methods (#1139)
 - Separated Wallclock time measurements from CPU time measurements and storing them under new 'cpu_time' variable (#1173)
-- Adapt RunHistory to be human readable (# 1174)
+- Adapt RunHistory to be human readable (#1174)
+- The models have a `is_trained` property to indicate the internal state (#1191)
 
 ## Dependencies
 - Allow numpy >= 2.x (#1146)
diff --git a/smac/model/abstract_model.py b/smac/model/abstract_model.py
index 18ee1bdda..cb7138d2d 100644
--- a/smac/model/abstract_model.py
+++ b/smac/model/abstract_model.py
@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from abc import abstractmethod
+from abc import ABC, abstractmethod
 from typing import Any, TypeVar
 
 import copy
@@ -19,14 +19,12 @@
 __copyright__ = "Copyright 2025, Leibniz University Hanover, Institute of AI"
 __license__ = "3-clause BSD"
 
-
 logger = get_logger(__name__)
 
-
 Self = TypeVar("Self", bound="AbstractModel")
 
 
-class AbstractModel:
+class AbstractModel(ABC):
     """Abstract implementation of the surrogate model.
 
     Note
@@ -57,6 +55,7 @@ def __init__(
         self._rng = np.random.RandomState(self._seed)
         self._instance_features = instance_features
         self._pca_components = pca_components
+        self._is_trained = False
 
         n_features = 0
         if self._instance_features is not None:
@@ -92,6 +91,12 @@ def meta(self) -> dict[str, Any]:
             "pca_components": self._pca_components,
         }
 
+    @property
+    @abstractmethod
+    def is_trained(self) -> bool:
+        """Returns True if the model is trained, False otherwise."""
+        raise NotImplementedError()  # make use of `self._is_trained` in subclasses
+
     def train(self: Self, X: np.ndarray, Y: np.ndarray) -> Self:
         """Trains the random forest on X and Y. Internally, calls the method `_train`.
 
diff --git a/smac/model/gaussian_process/abstract_gaussian_process.py b/smac/model/gaussian_process/abstract_gaussian_process.py
index 512817a38..ac8e945ff 100644
--- a/smac/model/gaussian_process/abstract_gaussian_process.py
+++ b/smac/model/gaussian_process/abstract_gaussian_process.py
@@ -58,6 +58,11 @@ def meta(self) -> dict[str, Any]:  # noqa: D102
 
         return meta
 
+    @property
+    def is_trained(self) -> bool:
+        """Returns whether the model is trained or not."""
+        return self._is_trained
+
     @abstractmethod
     def _get_gaussian_process(self) -> GaussianProcessRegressor:
         """Generates a Gaussian process."""
diff --git a/smac/model/gaussian_process/mcmc_gaussian_process.py b/smac/model/gaussian_process/mcmc_gaussian_process.py
index 71c4d45aa..c2d6f3ae7 100644
--- a/smac/model/gaussian_process/mcmc_gaussian_process.py
+++ b/smac/model/gaussian_process/mcmc_gaussian_process.py
@@ -287,7 +287,10 @@ def _train(
                 model.mean_y_ = self.mean_y_
                 model.std_y_ = self.std_y_
 
-        self._is_trained = True
+        if not self._models:
+            self._is_trained = False
+        else:
+            self._is_trained = all(model.is_trained for model in self._models)
         return self
 
     def _get_gaussian_process(self) -> GaussianProcessRegressor:
diff --git a/smac/model/multi_objective_model.py b/smac/model/multi_objective_model.py
index f4bf8c316..8b1a57634 100644
--- a/smac/model/multi_objective_model.py
+++ b/smac/model/multi_objective_model.py
@@ -53,6 +53,11 @@ def __init__(
             seed=seed,
         )
 
+    @property
+    def is_trained(self) -> bool:
+        """Whether the model is trained or not."""
+        return self._is_trained
+
     @property
     def models(self) -> list[AbstractModel]:
         """The internally used surrogate models."""
@@ -76,6 +81,8 @@ def _train(self: Self, X: np.ndarray, Y: np.ndarray) -> Self:
         for i, model in enumerate(self._models):
             model.train(X, Y[:, i])
 
+        self._is_trained = all(model.is_trained for model in self._models)
+
         return self
 
     def _predict(
diff --git a/smac/model/random_forest/abstract_random_forest.py b/smac/model/random_forest/abstract_random_forest.py
index 0703551f3..c328a9719 100644
--- a/smac/model/random_forest/abstract_random_forest.py
+++ b/smac/model/random_forest/abstract_random_forest.py
@@ -26,6 +26,11 @@ def __init__(self, *args: Any, **kwargs: Any) -> None:
         self._conditional: dict[int, bool] = dict()
         self._impute_values: dict[int, float] = dict()
 
+    @property
+    def is_trained(self) -> bool:
+        """Returns whether the model is trained or not."""
+        return self._is_trained
+
     def _impute_inactive(self, X: np.ndarray) -> np.ndarray:
         X = X.copy()
         for idx, hp in enumerate(list(self._configspace.values())):
diff --git a/smac/model/random_forest/random_forest.py b/smac/model/random_forest/random_forest.py
index 3500d03ce..3bd9fa99c 100644
--- a/smac/model/random_forest/random_forest.py
+++ b/smac/model/random_forest/random_forest.py
@@ -154,6 +154,8 @@ def _train(self, X: np.ndarray, y: np.ndarray) -> RandomForest:
         data = self._init_data_container(X, y)
         self._rf.fit(data, rng=self._rng)
 
+        self._is_trained = True
+
         return self
 
     def _init_data_container(self, X: np.ndarray, y: np.ndarray) -> DataContainer:
diff --git a/smac/model/random_model.py b/smac/model/random_model.py
index f55ce00bc..08a28b2f1 100644
--- a/smac/model/random_model.py
+++ b/smac/model/random_model.py
@@ -14,6 +14,11 @@
 class RandomModel(AbstractModel):
     """AbstractModel which returns random values on a call to `fit`."""
 
+    @property
+    def is_trained(self) -> bool:
+        """Returns whether the model is trained or not."""
+        return self._is_trained
+
     def _train(self, X: np.ndarray, Y: np.ndarray) -> RandomModel:
         if not isinstance(X, np.ndarray):
             raise NotImplementedError("X has to be of type np.ndarray.")
diff --git a/tests/test_model/test_abstract_model.py b/tests/test_model/test_abstract_model.py
index 6fe77e466..6b4613048 100644
--- a/tests/test_model/test_abstract_model.py
+++ b/tests/test_model/test_abstract_model.py
@@ -23,6 +23,16 @@ def get_X_y(cs, n_samples, n_instance_features):
 def _train(X, Y):
     return None
 
+class MyAbstractModel(AbstractModel):
+    # dummy class to ensure that the required methods are implemented
+    @property
+    def is_trained(self):
+        return self._is_trained
+
+    def _train(self, X, Y):
+        self._is_trained = True
+        return self
+
 
 def test_no_pca(configspace_small, make_scenario):
     n_instances = 100
@@ -35,7 +45,7 @@ def test_no_pca(configspace_small, make_scenario):
         n_instances=n_instances,
         n_instance_features=n_instance_features,
     )
-    model = AbstractModel(configspace_small, scenario.instance_features, pca_components=7)
+    model = MyAbstractModel(configspace_small, scenario.instance_features, pca_components=7)
     # We just overwrite the function as mock here
     model._train = _train
 
@@ -68,7 +78,7 @@ def test_pca(configspace_small, make_scenario):
         n_instances=n_instances,
         n_instance_features=n_instance_features,
     )
-    model = AbstractModel(configspace_small, scenario.instance_features, pca_components=7)
+    model = MyAbstractModel(configspace_small, scenario.instance_features, pca_components=7)
     # We just overwrite the function as mock here
     model._train = _train
 
@@ -88,3 +98,16 @@ def test_pca(configspace_small, make_scenario):
     X_test, _ = get_X_y(configspace_small, n_samples, 10)
     with pytest.raises(ValueError, match="Feature mismatch.*"):
         model.predict_marginalized(X_test)
+
+def test_abstract_model_raises_without_istrained(configspace_small):
+    class DummyModel(AbstractModel):
+        def _train(self, X: np.ndarray, Y: np.ndarray):
+            self._is_trained = True
+            return self
+
+        # @property  # -> because this code is missing, it should raise
+        # def is_trained(self) -> bool:
+        #     return self._is_trained
+
+    with pytest.raises(TypeError, match="instantiate abstract class"):
+        model = DummyModel(configspace_small)
diff --git a/tests/test_model/test_gp.py b/tests/test_model/test_gp.py
index ecdc8dd68..6bde0ec00 100644
--- a/tests/test_model/test_gp.py
+++ b/tests/test_model/test_gp.py
@@ -176,6 +176,16 @@ def test_predict():
         assert m_hat.shape == (10, 1)
         assert v_hat.shape == (10, 1)
 
+def test_is_trained():
+    seed = 1
+    rs = np.random.RandomState(seed)
+    X, Y, n_dims = get_cont_data(rs)
+
+    model = get_gp(n_dims, seed)
+    assert not model.is_trained
+    model._train(X[:10], Y[:10], optimize_hyperparameters=False)
+    assert model.is_trained
+
 
 def test_train_do_optimize():
     # Check that do_optimize does not mess with the kernel hyperparameters given to the Gaussian process!
diff --git a/tests/test_model/test_gp_mcmc.py b/tests/test_model/test_gp_mcmc.py
index ebba87dc7..f73a49804 100644
--- a/tests/test_model/test_gp_mcmc.py
+++ b/tests/test_model/test_gp_mcmc.py
@@ -91,8 +91,10 @@ def test_gp_train():
     fixture = np.array([0.693147, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, -6.907755])
 
     model = get_gp(10, seed)
+    assert not model.is_trained
     np.testing.assert_array_almost_equal(model._kernel.theta, fixture)
     model.train(X[:10], Y[:10])
+    assert model.is_trained
     assert len(model.models) == 36
 
     for base_model in model.models:
diff --git a/tests/test_model/test_rf.py b/tests/test_model/test_rf.py
index 65e6478ff..1b0532083 100644
--- a/tests/test_model/test_rf.py
+++ b/tests/test_model/test_rf.py
@@ -49,7 +49,9 @@ def test_predict():
     X = rs.rand(20, 10)
     Y = rs.rand(10, 1)
     model = RandomForest(configspace=_get_cs(10))
+    assert not model.is_trained
     model.train(X[:10], Y[:10])
+    assert model.is_trained
     m_hat, v_hat = model.predict(X[10:])
     assert m_hat.shape == (10, 1)
     assert v_hat.shape == (10, 1)