How can I identify and uninstall all apps installed by a certain user?
I think your best bet might be to come at it from the other end of the problem.

Open App Store, sign is as you if you didn't already, then check your own Purchased list.
Anything not in that list will belong to someone else.

Unfortunately, that will likely include the OS itself, which may cause future issues, especially if you don't know the old user's credentials, or if they ever change the password.

Wipe &amp; start afresh would have been better, in the long run.