Jeff,
I hold the stock and the receiver and shake the gun side to side and up and down. If it is loose it will almost certainly be off face. You can use feeler gauges between the breech and the barrels to measure how much of face.
Phil

P.S. A couple of good books to read: Vintage Guns for the Modern Shot by Diggory Hadoke and Double Guns and Custom Gunsmithing by Steve Dodd Hughes. Both are in print and readily available.