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Most importantly, code review is not a paper review. Its primary goal, almost always, is to make the code a better fit, better for future maintenance and useability.
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There is no one size fits all rules for code review.
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Projects may have different checklicks and review guides. Follow them when they exist.
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Generally, be nice. You may know the person who contributes, but there are others who may just read your interactions and don't know personal contexts.
Madicken Munk put together these good practices; consider following them:
https://munkm.github.io/research-software-schools/peer-review.slides.html#/
TL;DR -- Code reviewing is not trivial, be nice during the process.
Helen Kershaw (NCAR) has put together this Code Review Tutorial exercise. There are both Python and Fortran versions.