They called the little girl an angel more than once.

Also see where he locked one in a closet and it couldn't get out. Don't know if that's a demon thing or not, but he couldn't see it's reflection in the knife.

It turns out that Shyamalan, in an interview from 2006, has confirmed this.

"You know what I said about giving a Dracula movie a deceptive title? That's what I did with Signs. That was a story about a war between Heaven and Hell. The aliens were demons and the people's dead loved ones were angels."