What Rich said above. Especially the inside of the lock plate.
Also, look at the 'teeth' on the cock and top jaw that hold the flint. Are the cast in lines still there, or are there raised teeth from upsetting with a hammer and pointed chisel? If there are teeth, are they old looking,,, or new?
Look at the touch hole. If as usual a plug has been screwed in and then welded around, the welding is hard to hide; both texture and color. If that looks good, for a for sure determination, run a side looking bore scope down the barrel. And that will tell the tale for sure. I don't know of anyone, including me, that will try to disguise/blend in the interior of the barrel where the drum screwed in..
Either way, it's a very nice rifle!
John