I have never seen an authentic antique powderhorn in an antique store, but I have seen a lot of porly made modern ones from Mexico being foisted as antiques.
Look fine workmanship. Does the spout come to a graceful curve that is well finished and artistic. Or is it clunky and uneven? Next, is the plug in the large end set with wooden pegs around the rim of the horn or metal nailheads? Wooden pegs, ESPECIALLY if they are square, are common in old horns.