If you're really going with World of Warcraft (or D&D) style trolls, you've got a problem: They regenerate:
Although enough physical damage will kill them, trolls can regenerate lost limbs and heal grievous physical injuries at an accelerated rate, giving them a large advantage in battle.
So your proof is going to have to be something that doesn't regenerate. If you choose ears, or tusks, or fingers, or hands, or feet, your adventurers don't need to kill 15 trolls, they can capture just one, then harvest it for the relevant body part over and over (in both D&D and WoW, troll regeneration from nearly dead to unharmed occurs in the space of a few minutes at most, so you could harvest 15 trolls worth of body parts in the space of an hour, give or take).
So you need something that won't regenerate and can be uniquely associated with a single troll.
Unfortunately, the obvious solution here (in terms of maximum proof for minimum "amount of troll") is scalps; sure, the scalp itself can regenerate, but the hair is dead, and presumably grows at a relatively normal rate; you could harvest scalps over and over, but scalp #2 and onwards wouldn't have hair. The reason I say "unfortunately" is that nothing bleeds like a scalp wound and your adventurers are squeamish. As others have suggested, once the troll is dead, you could just wait a bit and the blood should coagulate, but if they don't want to deal with blood at all, they're stuck. Claws might also work (though in most depictions of trolls, a chopped off arm grows back with claws, so they may not be made of dead material the way human fingernails are).
The best I can come up with is having them collect the hair itself. It's not perfect; trolls have wildly varying amounts of, and growth patterns for, hair in most depictions, so it might be possible to take hair from one troll and pretend it came from two or more. But it's the best you're likely to come up with shy of dragging all 15 corpses back with you.
Ear wax, as mentioned in another answer, might also be viable (presumably carving out an ear and having it regrow won't have it regrow complete with ear wax build up), though as that answer notes, it can't give an accurate kill count, just a rough estimate (it's like hair, but even easier to subdivide).
There is a flaw with all of these methods: If trolls are smart, they might try to game your troll death metrics by regular grooming; shaving their heads, carving out the inner skin of their own ears (or use q-tips, whatever) every week or so, etc. If you're adventurers looking to make a buck, taking the risk of death fighting trolls that you won't be paid for might seem like a bad deal.
