What is the best way to check if a website is using all the JS code included?

I am working on a big website with lots of legacy code in it and I really would like to do some clean up as I know for sure that many JS libraries are included, but not used anymore. Obviously there is also custom code. Unfortunately they do not have any functional testing in place, no documentation and the team'members are all pretty new and no one seems to know anything, a nightmare.

I really do not understand how people can do something like that. There is really no respect or "programmers-code" like: do not leave shit behind you because another programmer like you will have to deal with it after!!!

Our job is awesome and I feel lucky to do it, but lack of respect makes me crazy.

How would you go about it? Do you know any tool that could help in this situations?