I know you get it checked by Blathers, but is there any way to tell BEFORE I buy it? Does Redd say anything different if it's fake? I'm just really tired of having only a few pieces of art in the museum. :s
I don't think you can tell without an online guide - the thonky one above is what I use. And I don't think you can tell anything by what your villagers say when they sell to you either. It's just the luck of the draw there.
the manual tells you the artworks used and their titles. if youre not sure, you could look up what it really looks like and see if the one in-game looks like it.