The first thing would simply be to see if the barrel is marked with a warning "use black powder only", or "black powder", something along those lines. IF not..., probably not a firing replica.
Second, if it is marked, you can obtain a flint and see if the lock will spark. It probably won't spark well if at all, and so then simply use it as a wall hanger for the cost of trying to get it to function will probably be too much.
Third IF it is a shootable barrel, AND it sparks fairly well, then use 2FG. A .69 caliber smooth bore uses 2Fg whether it's a Charleville musket or a .69 caliber, horse pistol, when firing a live projectile or shot. The ball would be .675 btw in a paper cartridge. 3Fg would be for blanks only in that pistol.
OH and you can't proof anything, sorry to say. Home-job-testing can be conducted but it's very crude compared to actual proof testing, done at a proof-house, and companies that say they "proof" black powder replica arms without sending the barrels out to such proof-houses, or who tell you how you can "proof" a barrel, are seriously misusing the term.
LD