Depending on the situation, you may be able to jump into the source code and check how they do things, but personally, I don't.
If the developer/community of a language, framework, library, etc. etc. haven't taken the time to write good documentation and provide some examples, I tend to stay away from it. It takes extra time for us to understand how something was built, and why. The developer should help us along with this.
Think about even the simplest API if it isn't documented, how long will it take you to figure out what you can do with it and how.
For me, it is also likely that the original developer is a much better programmer than I am. Or at least, I assume they are. I suspect there will be much of their source that I don't understand. Of course, reading their code makes me a better developer, but I'll often need help, and good documentation will help along with that.
It sounds horrible to say "I expect that if you're going to create an open-source bit of code that will help me code better/easier/faster, I also expect you to document it well and give examples", but if somebody is going through the trouble of developing something they want other people to use, I think this is an important part of that process.
What do people think of documenting first, and building from that documentation? I'm putting together my first open-source project, and am thinking that may be a good way to manage the documentation thing. Anybody done that before?