As Warr said, the best way to tell is to put it in 5th and if possible, be driving up a hill. Settle in at 3500 rpm (highest torque), and then floor it. Watch the rpm. It should spike up without a proportional increase in speed. And if you lift off the gas to say 3/4 or 1/2 throttle, the rpms will quickly drop back down while the car continues to accelerate. It should be pretty easy to tell. First time it happened to me on my first car, I knew exactly what it was. If there is any doubt though, logging would be the least subjective way to tell.