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.
(as a German I laugh about the naming of course. Munich Oktoberfest starts middle of September and ends first weekend of October)