Reading the source tends to be the most valuable way to assess the quality. Other factors like contribution frequency, ownership, contributing guidelines, documentation etc are important as well.

I think many people spend too little time on qualifying open-source dependencies or tools that they will use on a project. It'll do you a lot of good to dig into the source for an hour or so. If nothing else, read through the source that makes up the surface of the API that you are going to be using.