Document testing in the autoloading guide

Author: fxn

guides/source/autoloading_and_reloading_constants.md

@@ -456,6 +456,27 @@ However, if an engine supports Rails 6 or Rails 6.1 and does not control its par
 
 3. In `classic` mode, a file `app/model/concerns/foo.rb` is allowed to define both `Foo` and `Concerns::Foo`. In `zeitwerk` mode, there's only one option: it has to define `Foo`. In order to be compatible, define `Foo`.
 
+Testing
+-------
+
+### Manual Testing
+
+The task `zeitwerk:check` checks if the project tree follows the expected naming conventions and it is handy for manual checks. For example, if you're migrating from `classic` to `zeitwerk` mode, or if you're fixing something:
+
+```ruby
+% bin/rails zeitwerk:check
+Hold on, I am eager loading the application.
+All is good!
+```
+
+There can be additional output depending on the application configuration, but the last "All is good!" is what you are looking for.
+
+### Automated Testing
+
+It is a good practice to verify in the test suite that the project eager loads correctly.
+
+That covers Zeitwerk naming compliance and other possible error conditions. Please check the [section about testing eager loading](testing.html#testing-eager-loading) in the [_Testing Rails Applications_](testing.html) guide.
+
 Troubleshooting
 ---------------