There are almost no poisonous snakes. That’s because for something to be poisonous means it’s toxic if ingested. Like hemlock.
What many snakes are is venomous: able to inject a toxin into the body of their prey. But, there are actually two species of snakes that are poisonous and venomous. The Oregon common garter snake feeds on rough-skinned newts, which are poisonous to other creatures, and retains their toxins. Likewise, the Rhabdophis genus of snakes (commonly called keelback snakes) similarly feed on poisonous toads.
It’s the venomous snakes we’re usually concerned with. There are many guides online for identifying these dangerous reptiles. Like this one taken from WikiHow
This one from an American snake removal service
Or these ones from official North Carolina and Indiana government websites respectively
The problem is that even if we overlook the poisonous/venomous mistake, these are all incorrect. The claims that round pupils + no facial pits + skinny head = safe snake are just plain wrong.
The most dangerous snake in the world is the inland taipan. It’s found only in Australia, and its bite contains enough venom to kill at least 100 grown men. It also had round pupils and no facial pits.
Snakes’ pupil shape depends on the times of day in which they are active. Diurnal snakes (those active during the day) tend to have round pupils, while nocturnal snakes more often have slits. Slit pupils can help regulate the amount of light entering a snake’s eye, so that they aren’t blinded when operating in unexpected sunlight. They also help a snake to hunt by hiding the circular pupils that may stand out to prey.
The infrared thermal sensing facial pits in snakes developed not only to help snakes hunt, but also to help them thermoregulate. As cold-blooded creatures, snakes are very sensitive to changes in temperature. Their facial pits help them to seek out warm blooded prey, but also to find cool places to hide.
Facial pits are found only in the snake subfamilies Crotalinae (pit vipers), Boidae (boas) and Pythonidae (pythons). The previously mentioned inland taipan belongs to the subfamily Elapidae, which houses all five of the most venomous snakes. None of which have facial pits (shown with red arrows in the picture below).
What about the skinny vs fat head? Most wide snake heads are so because they contain the venom glands. But, while a fat-headed snake is more likely to be venomous, there are still plenty of skinny headed snakes that keep their venom elsewhere.
How can you tell if a snake if venomous? Call an expert. Even if there was a reliable way to recognize venomous snakes, I can’t recommend getting close enough to a wild snake to examine it.
Luckily, at least here in Canada, you’re unlikely to encounter a venomous snake. There are only four venomous species found natively in the True North, and none of them are found in Quebec.
But please, if you everget bit by a snake, go to the hospital. It’s always better to be safe than sorry.