The size? Elephants are much larger than other animals they’d come in contact with, except for rhinos and hippos, and even then, they are larger by a good margin.
I think it would be pretty easy to tell human bones apart from animal bones. Our bones are much straighter, thinner, and just different from four-legged creatures. The only skeletons that look remotely like ours are other apes, and even then, their legs are more bowed and their skulls are way different.
That’s true for an entire articulated skeleton. But long limb bones
of deer or sheep can look very much like human limb bones.
Ditto random vertebrae, ribs, phalanges, teeth, etc. Bones tend to
be scattered by scavengers & water, so the context of the entire
carcass would usually be absent. & hydrocephalic calves’ skulls can
look a lot like partial human craniums.
Living in a rural area growing up and playing in the woods pretty much every day, I came across a few carcasses of animals. More often, however, I came across skulls and other bones. A lot of rural folks have animal skulls they found in the woods as decorations (at least where I come from).