Cottonmouths/Water Moccasins are pit vipers and have a very noticeable sensory pit organ between their eyes and nostrils.

Pit viper's heads are "triangular" and wider than the neck because of the large venom glands and muscle for squeezing the gland.

Pit vipers have vertical pupils.

The triangular head is really the most noticeable way to identify a pit viper at a distance... Unless of course it's a Rattlesnake and making noise or it's a Cottonmouth showing the threat display of a wide open mouth (which a few snakes mimic).