No "already cleaned out" message on pickpocketing
Impossible to determine if a monster can still be picked.
After the first success or "doesn't seem to own anything of value", further pickpocketing attempts always result in a message of "you fail badly" (I've tried this on dozens of gremlins with a Pickpocket skill at 50). I remember there used to be a message along the lines of "You have already cleaned out the <monster>."
In fact, I also think there was some chance of failing the pickpocket attempt without "failing badly" - but that might only apply to non-hostile monsters, since it only affects whether the creature is alerted to your attempt.