The only way to test it correctly is if it has a manual valve body. If it does, put it in high gear. Set the parking brake, hold the brake pedal firmly. Now ease into the gas watching the tach. When the tach stops climbing, that's your stall speed. You can do it in first gear if it's not a manual valve body, but your result will not be accurate. I would suspect you could subtract at least 500 RPM from that result to be "close". But that's just a guess.
