How do you tell if an imported library ever gets use by the end user?

I'm pretty new to software but I have a strong stats background and naturally I want everything to tell me how useful it really is.

At my work we have a pretty large web app that makes calls for big files. I would really like to start paring down on what is called and used but I am not sure where to start because I am not sure what files are actually getting used.

We have millions of visitors that use our app every month. We are using Angular 1 and Foundations along with a bunch of other things. If it is possible I would like to track if a foundations or angular library is requested, the size of the file, and if it was used.

Has anyone done something similar to this before? I know how to check and see if a function is used but I'm not sure the best way to get started on something like this.

Sorry in advance for the noob comment. Give me a few years haha.