Hi in the modeler you can navigate to Edit > Find Advanced > Search on document > document type: Java action
This will list all the java actions used in the app.
For removing old/unused libs there are a couple of ways, but one is this: GitHub - cinaq/mendix-userlib-cleaner: A simple CLI tool that identifies duplicate JARS in a directory. It can remove them also if desired.
It is a tool that removes duplicate jars. Be carefull to back-up your old Jars since sometimes too much is deleted.