There is no "single cue" to tell a Cottonmouth from a harmless watersnake. There are a lot of urban legend type ways, but they aren't really dependable.

Here are some of the things you may have heard/read:

1. The shape of the head. Yes, a Cottonmouth's head is generally more triangular than most watersnakes, but actually from top to bottom, not from side to side as most people assume (i.e. it is more triangular when seen from the side, not necessarily from the top). Harmless watersnakes do generally have more narrow heads, but when they are disturbed, they flatten their heads out making them look triangular. So a scared/annoyed harmless watersnake has just as "triangular" a head as a cottonmouth.

2. The pupil of a cottonmouth is cat-like (elliptical). This is a great key except that if you really look at a cat (or cottonmouth's) pupil you will see that it is only elliptical in bright light. In dim light, it is round. This is not a safe way to tell them apart.

3. Behavior. I've heard all the old wive's tails about Cottonmouth's behaving differently than watersnakes. I've also spent hundreds of real hours out in the field working directly with both Cottonmouths and Watersnakes as part of several different research programs. I can safely say I've seen many hundreds of Cottonmouths and probably thousands of watersnakes (of at least 7 different species) and I can tell you with all my experience, I would never rely on the snake's behavior to identify it.

4. Range. People report seeing Cottonmouths well outside of their range all the time. I hear people talking about finding Cottonmouths (Water Mocassins) in the Northeastern US (where they don't occur) and even on the west coast (where they also don't occur). If you don't live in Cottonmouth range, you probably don't need to worry about them.

I was giving a talk once with one of the world's most famous herpetologists and a person in the audience asked him how you tell venomous from non-venomous snakes. I think his answer was perfect - "If you are using nursery rhymes [or other simple 'rules'] to help you tell a venomous from a non-venomous snakes, you probably shouldn't be messing with them at all."

Telling Cottonmouths from harmless watersnakes requires some experience and expertise. You can't find a single rule that tells them apart..

Imagine, for example, if pickup trucks didn't have metal names or numbers on them telling you what brand they were. Try to come up with one simple rule that would help someone tell a Ford pickup from a Dodge, Chevrolet, a Toyota, etc.. You couldn't.

Same with snakes. You either know how to tell them apart, or you don't. If you don't, leave them all alone.

Lastly (and I know I'm on a soapbox here and not really answering your question), remember that Cottonmouths aren't aggressive or really that dangerous. You pretty much have to do something stupid to get bitten by one (other than stepping on one that is unseen as happened to someone I knew). They aren't aggressive, they don't chase people, they don't drop out of trees onto boats, they don't get in big mating groups for people to waterski into, etc. . They are the victim of more undeserved urban legends than any other species of snake in the US..

Appreciate them for their uniqueness and the fact that unlike the Bald Eagle, they only occur in the US.!

They are amazing beasts, deserve our respect, but certainly not our fear.

When I took this photo, this big fella didn't even try to bite me even though I was within a foot of him (don't try that at home, BTW. I have decades of experience with the species.)

Great information, FHFchrish. Thank you for taking the time to write that!