How can an administrator find the UID of a user that's been deleted with the
By looking at
If there are no backups, and no obvious files owned by that user you can check, then you'll need to scout around places like
Edit: as jw013 points out you could also use
If there are 10 users and 1 deleted user, you could check /etc/passwd for which UID from 1000 and up is missing.