He should know it's 4WD from looking underneath if he's not a complete loon!
However, they don't actually drive the cars on the brake test your thinking of a RR. On a brake test its the rollers that spin and the machine tests your cars brakes ability to stop them turning. It's advisable that you don't let them test the car on the rollers anyway and get them to use the incar gadget and take it around the block to save your diff.