What to look for in a code review?

What are your go-to resources for “What to look for in a code review?”
I’m thinking of a range of audiences from people who’ve never contributed to a code review before to people who have but maybe feel like they’re missing something.

I’m not in a position where I do code reviews currently but in the past I always looked at the code review comments for any item I was going to test not only to check what had been flagged up had actually been fixed but the kinds of problems that were being found. I always thought this informed my testing more, gave context to potentially something I might not have realised was being changed and maybe some gaps in the PBI/story/item that needed more attention.