Dwayne Johnson's Workout -- How Much Does The Rock Bench? ... So High School

When ya see a guy the size of Dwayne "The Rock" Johnson … one obvious question pops in your head, but whatever you do -- do NOT ask it out loud … as our camera guy learned.

The Rock -- so damn huge he's playing "Hercules" -- was in NYC Tuesday when our pap fell into gym meathead mode and asked, "How much are you benching?"

You gotta see Rock's initial reaction to the question (which he must hear 100 times/day) -- but he does eventually hit the photog with the perfect answer … for the smartass question.

Our guy shoulda learned from Alex Pettyfer -- who made the same screwup on Muscle Beach last weekend.