Another way to know without pulling the locks is to count the screw heads between the trigger guard and the hinge on the bottom of the action. There will be two on this gun (boxlock), vs one on a sidelock.
I have only seen one vendor who could fake that one out (Dumoulin).
Yet another way is to look and see if the tumbler axis rotates while the gun is cocked and upon (dry) firing.
However, some of the fake sidelocks have a fake tumbler axis that rotates when the gun is being cocked and fired. At that stage it's almost cheaper to go for a real sidelock!
WC-
