Forgery

A Forged Painting is the outcome of buying a fake painting from Crazy Redd. A forged painting appears as a regular painting until sold to Tom Nook or donated to Blathers at the Museum, where both may identify the picture.

Identifying a fake painting
Contrary to popular belief, it is not possible to identify whether a painting is a forgery or not simply by Redd's emotions and dialogue. Indeed, in WW, CF and earlier games, it is not possible to tell the authenticity of a painting until the player buys it and takes it to Blathers in the museum. Sometimes a city villager will mention a fake painting.

Identifying a fake painting  in Animal Crossing: New Leaf
In NL, it is possible to determine whether or not a painting is forged by simple visual inspection. One or more areas of the painting will have a noticeable difference from the real painting.