Front is easy.
For the rear, jack up the car at the differential and then put the jack stands under the rubber pads below the jack holes.
Several of us have done the tests. If you just want to fill the differential properly, you do not HAVE TO have the car in a perfectly level situation. You can park the car on a slight slope (front higher) and drive the car up to a pair of ramps in the rear. Fill the differential and you will be surprised how much it takes. Exactly the capacity about 1.2 qts.
If you take a look at the pic, the extra fluid in the lower portion of the plug compensates with the lack of in the higher portion of the plug. The result is that the fill capacity does not have too much to do with whether the car is level or not.
I have done differential fluid changes 8 times on MBs this way and it always takes what the capacity is.