How to find source code for events that trigger features on web apps

I am just getting my beak wet with web development. I found a site navigation bar on http://ift.tt/2apZI39 that I really liked and wanted to emulate it's interactivity, but I cannot find the code that triggers. It is triggers by scrolling on the page and I've managed to do it in my own way, but I am still curious as to whether there is a way to find the javaScript code that catches the scroll event. Again, I very new to web development and was curious as to whether there are tools that will allow me to catch the JS code on foreign web app.