Generally you should get a basic feel from looking at the bug trackers and other community resources. Bad documentation you should be able to spot by looking at it or notice early when trying to follow it.
Do they have clear instructions on how to get started, run tests etc? How's the tone? What are discussions on PRs like, is there good feedback or are PRs from non-core people just ignored?
IMHO that's something you have to judge yourself, if you feel comfortable with a project and what kind of resources you need. (E.g. for me, many javascript projects are hard to contribute to because I have little experience with common tooling. But at the same time I know that in the JS community, most people are familiar with them and would not see this as a hurdle, and I would not necessarily fault the project for it.)
Some projects explicitly mark issues as "beginner friendly", "good first project" etc.
And try to pick something you use or care about: It's a lot harder to stay motivated if you don't really care about the end result.
