What 2 traits are used to classify venomous snakes?

One is by the shape of the head, such as the bit vipers, which have triangular shape with it's head being larder then it's neck.

The other is for the coral snake, which can be identified by it's yellow ring touching the red rind. Milk snakes, which are not that poisonous, has the same colour combination as the coral snake but but it's red rings does not touch it's yellow ring.

A mnemonic that helps me tell the difference between a coral snake and milk snake if "If yellow touches red, then your dead."

Also, if you look at the snakes eyes, you will notice that venomous snake have elliptic eyes verses non-venomous snakes having round eyes.