You cannot tell, unless they've done something relaly radical like a voltmod, or BIOS flash.
If they just use ATItool, you'll never find anything to hint that they've been overclocking.
Telltale signs of overclocking is an aftermarket cooler, although it may be for noise reasons.
But then again, it might mean they know what they're doing. Compared to someone who just discovered something called ATItool off google when he typed in "overclock", and decided to see how high he can go. On stock cooling.
Might be a good thing too, cos he'll crash the card, and probably sell it off ASAP, cheap, because he thinks he's permanently damaged the card.