to detect clutch slip it is often easiest to find a long fairly steep hill and try to go up it in 4th gear at about 30mph. if the revs rise and speed doesn't increase you have slipping. if the revs drop and the car stuggles up the hill then the clutch is fine.
when i inspect the timing belt i look for cracking, and i want to see square edged teeth on the belt, and i check that the belt inbetween the teeth does not look as though it has stretched.
think its a good time to point out that the auxilliary drive belt is ribbed, not toothed like the timing belt. on a ribbed belt it is perfectly normal to have cracking on the inside ribs of the belt, even a fairly new one will have cracks, but a toothed belt such as a timing belt should have no cracking at all.