I believe it's the markings on the top of the head (kinda like how fire's have the lighter markings too) and the nose of course, at least that's what I look for. From what I see they tend to (usually) have busy body patterns too, kinda jungle-y. Here's some good examples:
The markings on their head are the most noticeable, and they remind me of the markings ont he top of a carpet python's head. More subtle is in their body pattern, which reminds me of a spider, but not as extreme, at least in really good examples of the morph. Kind makes me think of a halfway point between a normal and a spider... and something I didn't really notice until I saw one in person.