Just about all poisonous snakes in N America have a vertical looking pupil. Just google a rattler, cottonmouth and copperhead and see for yourself. The Corral snake may be the exception. Most of us don't want to get close enough to verify...
They also have a pit between their eyes and nostrils hence the name "pit vipers". But again, who wants to get close enough to see that? I have learned that a water snake when feeling threatened, can flatten its head and it appears triangular, like a cottonmouth. Markings can be similar so, again, the pupil and pits are dead giveaways. So feel free to get nose to nose to examine his pupils and nostrils, lol!
shadow LIKED above post