When uninstalling a program, which folder(s) should you check for left behind files?

Well, it really depends on the app itself. For most programs, everything is contained in the app package itself, aside from the .plist files, which are in the Library/Preferences folder as you mentioned (keep in mind there are 2 libraries - a system library and a user library, and they may both have .plist files related to the app).

Some programs - notoriously the Office apps - distribute little files absolutely everywhere, and it's horrendous. Programs like AppZapper are supposed to automatically remove all those files, but I can't personally vouch for them.

If you're worried about space, keep in mind .plist files and other support files are generally kilobytes in size, which is negligible.

 

/Library/Application Support/ and the equivalent user folder is the only real place you need to look for most things.

 

^^^


This. And a search with finder should reveal any leftovers that may have been missed. Some apps come with an uninstall routine but I always double check that nothing got left over.

 

/Library/ & /Users/Username/Library/ respectively and I use AppCleaner to get rid of the leftovers on uninstall it works well.

 

The most effective method for complete app removal is manual deletion:

Best way to FULLY DELETE a program​

If you elect to use such apps, be aware that in most cases, app removal software doesn't do a thorough job of finding and removing files/folders related to deleted apps. For more information, read this and this. If you just want to delete the app, drag the .app file to the trash. No other software needed. If you want to completely remove all associated files/folders, no removal apps will do the job.

This is frequently not the case, as many apps have related files that can be in excess of 1GB.

 

Since that method was what I was going to suggest cool, I guess you probably have some limited knowledge in this area.