Most of the time, if you are working on a well defined project, it's pretty easy to find where a function is if you want to change/tweak it. You can look in the class where it is defined and/or see if there are any mixins or decorators.
Sometimes, it can be downright impossible to tell.
Fire up your rails console and put in the following (for a class method :search on User)
if you have an instance method
This should give you something like the following
This should at least give you some idea of what is going on...