It was easier with old car: a normal diff would spin the wheels opposite ways in gear with the engine off. A clutch-type would spin them the same direction in neutral, and probably not at all in gear. With a torsen, you're out of luck. In the car, driving is an easy test: redline first, and take a hard (left!) corner as tight as you can. If the inside wheel spins, you have an open diff. Also, if you can lay down one strip of rubber, odds are you have an open diff.
