If both wheels spin, the clutches/sliders are good enough to hold with the wheels in the air, and almost zero load.
Not necessarily 100% or even close to it for that matter. Puttin the horse back on the ground, and putting all the other horses through the driveshaft may say the clutches/sliders are worn out, but it still might have enough grab for this test. On the other hand, if the posi won't grab with the wheels in the air, then it's DEFINITELY worn out. With me?
As far as the test goes, if both wheels spin, it's a very easy way to figure out what gears are back there. I always mark the driveshaft with a grease pencil, inline with a landmark on the diff. case. Works pretty good!