[Joe T]

If you can only do a visual inspection prior to purchase,
check the oil level, and see visually what the oil is like. if you get enough out stick a magnet in it for particles.
Try turning the spigot shaft by hand check for lost movement before output shafts turn.
Check the clutch splines on the spigot shaft, could give an indication to usage, any steps in the end of the shiney part of the splines etc.
Check the end of the spigot shaft for burrs or damage.
Try selecting some gears.

Cheers

Joe T