I agree that to get a real caster reading, you would need the body on the car, but in this case, you're only concerned with whether it is positive or negative. On my camber gauge, you determine if the caster is positive or negative by determining the "polarity" of the difference of the camber when the wheel is turned in 20 deg and turned out 20 deg. I would think that doing that with the suspension even close to where it will be when the car is on the ground would be close enough to tell you if it's positive or negative.