If you have both rear wheels in the air and don't engage the parking brake, when you rotate one wheel the other wheel spins in the other direction, the differential is doing its job.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.