Demons, sometimes, doesn't look like a human. But they can appear as anything. They usually appears as little kids, because people usually opens arms for little kids instead of standing on guard. They used this as bait.
On way of telling is through the history. If no children died there, than it is probably not children. Even if children died there, it could still be a demon. In the "Dark World" by Zak Bagans, he says "Halles-Bar Marina in Tennessee has a graveyard full of children on the property, but the spirits inside are anything but innocent...It's a place where I thought I was going to comfort children, but instead got attacked."
Demons can also be invited to the place. If the history of the place has devil worshippers, than it might be a demon or an evil ghost. Demons aren't always invited by devil worshipping. Things like Ouija boards can open a portal to them as well.
If you get scratched in threes,it is said to be from a demonic being. It symbolizes mockery of the holy trinity.