You can't. The only exception is when he's selling the Awesome Painting, which is always fake (it should have been called Amazing Painting but they gave it the wrong name).
And before anybody points it out, no, you can't tell if it's fake or not by looking at a "star" that appears over Redd's eyes or anything. --- ~I am Dyslexic of Gorb. Prerape to be ass laminated.~ Proud non-owner of a Myspace account.