At the beginning of the debate, both candidates shake hands and exchange "pleasantries". When Kerry comes to shake Duh's hand, he casually pats Duh on the back with his left hand during the "pleasantries". If he detects any lumps or bumps, he could make a remark at the beginning enquiring about the pResident's health, noting that there appears to be some kind of "growth" on Duh's back.
Then watch the squirming begin....