How do I find all the ruby/rails app gem calls for specific gem?

So, I am working on a large app for a company with many gems. The app needs a lot of upgrading and part of the job is upgrading the gems. The upgrade itself is simple enough but the problem is testing to see if the upgrades changed or hurt the functionality of the existing app.

I need some method/process to when given specific gems, I can find in the code where do I use this gem and call its methods and classes.

Does such process exist?

Thanks for any help!