Been there, and it stings for weeks. The paper trick that gets me every time is the feel of it, because old paper is soft and slightly fuzzy while reprints are slick and stiff, like a new dollar bill. Take a flashlight and look at the surface at a low angle, since a real old print shows uneven ink and little dots of texture while a modern one looks flat and even. Check the edges too, because hand cut or rough edges from that era almost never come out perfectly straight, and 11x17 is a dead giveaway since nobody was printing in that size in 1847. Also flip it over, because a lot of those gift shop copies have a faint barcode or a tiny copyright line hiding near the bottom. Next time ask the seller to let you hold it up to the light before money changes hands, and if they get weird about it, walk.