Sometimes when you buy Paintings from Redd's shop it can be fake. There's another way instead of talking to Blathers to tell if it's real or not.

If you want to buy the Painting from Redd he'll sometimes something about "paws and knees". If he does that mean it's real, so buy it. A way to tell if it's fake is if he does the "mischeif" emoticon or says nothing about "paws on my knees".
I can tell because he said that weird quote and I bought it, I donated it to the Museum and Blathers said it was real.