How about the join date at the top of their personal profiles? Other clues, but not necessarily proof that the user is new, are: default jellyfish avatars, improper use of Fluther’s formatting standard called textile, low lurve scores, txtspeek (or other clues that the user hasn’t read the guidelines yet).
Disclaimer, these are only clues that the specific account is new. It doesn’t mean the user hasn’t been browsing Fluther without an account for weeks already (I did for a week or two), or that the new account is an older user’s new or alternative account.
edit: Basically everything @Jeruba said in her first answer.. :)