I learned it by doing more code reviews (and reading code reviews from others). I think there are no strict “guidelines”, every review is different (and maybe needs additional things to look at). The short answer is: “Just check if you are happy with the code/changes, because you are the one that needs to live/work with the code in the future.” That’s why a assign you for reviews where I think it’s useful when you have a look at them (but you’re welcome to also review other stuff ).
Here are some things I do when reviewing a PR:
First I read and (try to) understand the code. This is easier when the PR is separated in multiple commits with messages which describe what was done and why (or with a PR description containing the same). And this is also easier when refactorings are in their own commit. If I don’t understand something, I ask (I already learned some stuff by reviewing your PRs ;)).
While reading I always check if I would have done something different than the PR author did. This doesn’t mean that my solution would be better, so I need to consider, if the solution from the PR is OK, or if my solution has any advantages. If I think the code can improve with my solution, I ask (maybe the PR-author has done it that way because of a reasen, that I didn’t think about).
Also while reading, I can sometimes already spot things that maybe breaks in an edge case. I keep that in mind so I can test that later when running the code. And the better you know the code the better you know where you have to look for possible pitfalls.
Then I check if there are tests for new code, and if there are tests needed (sometimes things can’t be tested). And I check if the tests test useful things
For PRs with multiple review rounds I only check the updates between the rounds, but at the end I always do a full review again (maybe there is stuff that is obsolete now, but was missed).
I know that feeling, and reviews are not easy. That’s why it sometimes takes time until you get a review But in the end, code reviews are no guarantee to find everything. So nobody gets angry if you approved a PR, but later a problem is detected with that code. People make mistakes.
So just continue doing reviews and you will improve (and also your own PRs probably will improve by reviewing others PRs)