Yea there is a little chip on the back of the cluster visable through an opening in the white plastic. A good indication that the chip was swapped would be that the plastic piece that seals the chip into the cluster (its a brownish-off white color) is busted. So if its not broken and you know the cluster is not the one that came with the car, you can probably bet that mileage is for another stang!
I don't know if there is any other way to tell if the entire unit was simply swapped with a cluster from another car. You might be able to find a bar code/number on the back of the cluster. Look for stripping in the screws also, since there are only a few reasons to take the cluster off in the first place, and they are a pain to unscrew sometimes.
Other than that, if you think there is an odometer problem, steer clear. Thats a huge problem that you don't even want to have to deal with!