Fortunately with these bikes, it really doesn't matter if it has "rolled" or not from a longetivity standpoint. Unless, of course, you are buying RJ's or DDT's bikes - which have "rolled" multiple times.

Of course, it matters if you are the one buying a bike as the price would be dramatically reduced if you could prove the odometer has been "rolled".
Another item to check would be the wheels. They frequently get overlooked when the bike is being prepped for sale. If the wheels have signs of wear, chances are it's got 100K more miles than indicated.