I'm a beginning programmer, that for various reasons is using an existing Ruby codebase to learn to program. My goal is to be able to understand and eventually extend this codebase. However, I find it hard to understand what is a standard library function, a gem function, or a user-created function. Distinguishing between functions may not be the only thing that I need to do, there might be other "thingies" moving around too. The reason for the question is that I want to be able to read the code, and as I go through identify what documentation I need to read in order to understand the code.
Hence, my question is: in Ruby code, how do I distinguish between things belonging to the standard library, specific gems, and those that are user-generated, so that I may look in the right place to understand what identified functions (and other things) does?