I find it useful to open the code in an editor and make running notes in the comments about what I think the state should be. As long as the code has good tests

I find it useful to open the code in an editor and make running notes in the comments about what I think the state should be. As long as the code has good tests you can use debugging statements to confirm your understanding. As a bonus you can just send that whole block of code - notes and all - to a colleague if you get stuck. They can read through the code and your thoughts and give feedback.