Like Mr. Forney, I have found quite a few ML's that had been left loaded for many decades. When folks found out that I could clean/refurbish the old "wall-hangers", I had almost a line to provide that service. And about half of the old ones still had a charge in them.
First thing I would do would be remove any primer cap (most were percussion cap guns), then slide the RR down the barrel and see how far it goes. Usually lack about 2" or so of reaching where the breech was - determined by the drum. At first I did water injection to "neutralize" the BP, but after a while, on #10 or 12 or so, I would just try to fire them. Usually either pull the nipple or open the clean-out screw, trickle in a wee bit of 3F, new cap, and fire. And no, I didn't hold it - I had some surgical tubing and would lash it to my lead sled, string to the trigger, and fire. And, MOST of the time, they would go off.
If not, then pull the bullet, flush out the powder charge with soapy cold water, then commence cleaning. And most of the time they could be "restored" to be a useable ML rifle or shotgun. If not, a few customers had me plug the nipple base to preclude firing. Had one from the early 1840's - old Brit double shotgun, 10-bore. The owner said even his Granddad just had it hanging on the wall, in the family since new. Wound up being very shootable, and the owner took a Canada goose with a load of #4 bismuth shot.