Needed fast - rooted or not? topic

Hi guys,
I'm a soon to be Xperiaman and it's my first time to own a Sony phone. I'm buying it secondhanded and it is still under warranty - how can I check if it's rooted or if there have been any custom ROMs installed? Is there a custom binary counter or something alike? If not, is there a way to make it invisible for a dealer to see if the phone has been rooted/modified in software?
I'm buying the phone in 10 hours so any quick help is appreciated.