Steve's signature is so rare that its hard to ever know. Other than signed official stamped documents like the founding papers that just sold at auction how would you ever know for sure?
I remember reading a story about how he went to a friends dinner party. The host who knew steve well asked if he would sign his old apple keyboard. Steve went on a rant on how ugly the keyboard was and how it was old in design. I dont think he ever signed it 😉