I thought this magic malarky was all about secrets!
edit]

heh, heh, heh...
Alright my idea is that said shirt has a picture of a card or something on the back, then after ******** that something on the spec and failing to find that it the hapless magician throws his arms up in the air and turns away saying I quit! tadah!!! instant comedy revelation
Or even better, it's on the inside of the shirt above the heart, so you get specy to put their hand over your heart and mentally send you the image, jump back and after revealing it verbally, lift the shirt proving that they really did send it using the powers of their brain!!
I really like these ideas...