One thing that can often help with this is to have a good REPL. With it you can dive into and explore the code as it is at runtime. For example, a common way that I figured out Rails was to open the console and use obj.methods to see what methods are available.

Other languages in which I've enjoyed REPLs include Python, JavaScript and Haskell. Two languages I have not found a good REPL for are Java and C++. Is anything of this sort available there?