Pretty sure he's not looking up the reflection in the glasses (angle doesn't work and he's looking at bright background). If I got it right, it's actually much simpler than that. Make sure you clean you glasses.
This trick would certainly be acceptable on Fool Us, but Penn and Teller already know it: they did a version of it on the last Fool Us show, where they tried to fool Mac King and Piff the Magic Dragon but didn't succeed.
