Eightbore is right. Off face can only be measured accurately by trying to close a double with a shim in place. And both barrels should be tested because using the shim test one barrel could indicate it is off face but the other barrel might be solidly on face. The wiggle test mentioned earlier is a poor gauge because if the barrel lug is a snug fit into the action recess it might show no movement side to side but still be off face. On face means the breech ends of the barrels are smack up against the face of the action.
