Well, this time around in New Leaf, it appears that there is a way to tell if you're looking at a forgery or an authentic piece. In the Animal Crossing New Leaf Guide
by Prima, Chapter 14 - Museum Donations, has pictures of what the authentic pieces of art and sculptures look like. When you're in Redd's black market tent of potential goodies, look at the picture in the book for the item and compare it to the one that he's trying to sell you. If you can't find any differences, grab it up; but if you do spot a difference -- and sometimes they're really subtle -- move on to the next piece and don't waste your bells on it.
Of course, if you like the mystery and intrigue of picking up an piece of art and not knowing whether it will be accepted by Blathers or if he'll kindly reject it when you try to donate, don't bother with the guide -- it'll pretty much be the same experience you've had with the older versions of Animal Crossing. Oh, and one other thing -- be sure to check and make sure that the item you're about to purchase/donate has not been already donated to the museum. In previous versions of Animal Crossing, if you tried to donate an item that had already been donated, Blathers would reject your donation, while politely accusing you of producing counterfeit items. I haven't experienced this yet in New Leaf, but I don't have any reason to think it wouldn't be the same. Have fun building your museum!