A lot of it is guesswork, I think. Unless you put breakpoints everywhere to see which files are being accessed at what time, it can be quite a struggle to figure out which .JS file is meant for whichever functionality.
It depends, though. If you see a slider, you can expect the CSS styles and classes, and then when you see something like
class="superSlider", and a .JS file called superSlider.js, you’d know where to look.