You can't really tell without driving the car. Clutch wear varies widely because it is mostly dependent on how you treat the clutch. Some people report getting 150,000 miles on the original clutch (I don't know how true that is) but it is certainly possible if you never slip it and drive very conservatively or do 99% highway driving.
If you get a chance to drive the car, shift normally up to about 35-40 mph. Shift into 5th gear. Floor it. The car should not accelerate very quickly and the revs should build just as slowly as the car's speed. If the clutch is going to slip, now is the time and you'll feel it happening as the revs build faster than the car's speed.
However, even if the clutch is in great shape, if you plan on driving the car aggressively (and you probably are, why else would you buy an E39 M5 in this day and age) you will probably have to replace the clutch soon anyway. The factory clutch doesn't last too long when it's abused. Maybe 40k. My car is on it's 3rd OEM clutch at 102k miles (I bought it with 86k on it, already had the 3rd clutch in it) and this one isn't going to last much longer even though I do baby the car alot. The previous owner was pretty hard on the car so I anticipate the 4th clutch will last more like 80k miles since I won't abuse it.
Good luck with the purchase though, these are amazing cars and most of the factory hardware is pretty robust.