Image credit: Aaron Deutsch
In the last few months, I've been doing a lot of Drupal core contributions and, even though I've done plenty of contributions over the years, I've recently learned some new things when reviewing issues. I've made this cheat sheet for anyone who wants to help review Drupal issues and wants to make sure everything has been thoroughly checked before marking "RTBC" (Reviewed & tested by the community).
There is plenty of great documentation on the development contribution process that explains some of these items further which I've also linked to below. Documentation is also being improved right now so I will update this post as new information is available. For now, hopefully this post can serve as a handy reference.