The common practice is for the app developers to provide an uninstall-xyz.pkg
, especially if the app installs some privileged helpers or other files outside of its /Applications/AppName.app
folder.
If there are no uninstall packages or scripts provided by the app developer, then you can unpack the pkg file and see what's inside it. An easier alternative is to use an app that can peek inside a .pkg
and show you what files are copied where and what pre- and post-install scripts are run. This can be done for example with SuspiciousPackage (freeware) or Pacifist (nagware).
With SuspiciousPackage pressing Space (or QuickView) in Finder on a .pkg
file will show something like this:

From this screen you can further navigate to inspect the files in the package as well as the preinstall/postinstall
scripts.
Additionally, Pacifist has an option to Analyze Existing Installations which will let you inspect what a package had installed even if you don't have the original .pkg
at hand.
.pkg
files without a corresponding uninstaller, please contact the developer and ask them.