1) Sighting. You actually saw the animal. Either with your own eyes or on a wildlife camera.
2)Sound. You heard the raptors scree or the coyotes night song.
3)Sign. The bear stripped the bark off the tree from twelve feet up to the ground. The woodpecker left a thousand holes in that tree.
And of course one of the most useful ones.
4)Scat. The animal pooped there.
Well according to #4 the bats who I saw weeks ago and heard last week, were over every single car in the parking complex.
There is simply bat guano everywhere.