What are the signs of venomous snakes?
With the exception in the US of the coral snake which kills by a neurotoxin and does not have the head shape of other venomous snakes— they are called pit vipers for the pits behind their eye ridges on the top of the head. Virtually every venomous snake in the US is a pit viper.
In Australia in terms of degrees of deadly— the top of the list is the Australian Inland Taipan, followed by the Eastern Brown Snake— these being the two top venomous snakes in the world.
A venomous snake is defined as a snake producing a toxin venom for killing prey (and/or digesting them as well). Calling them poisonous is a misnomer as poison has to be ingested by someone to be active— venom is injected, and typically by fangs (of various size).
World wide there are about 600 varieties of venomous snakes. Venom is either neuro-toxic (paralysis) or hemotoxic (blood toxins). Interestingly major human pharmaceuticals and anesthesia agents have been derived from snake venom.
Learned a lot in toxicology classes back when. Had a friend who raised and milked King Cobras (no freaking way!!).
Google it. You will find pictures. Three of the four in the USA have big heads that extend cheeks past their body lines- nonpoisonous snakes have heads that do not stick out wider than the body. Coral snakes, well you just have to look at the picture then unless you spend a lot of time on memorizing what the color pattern is avoid all the black and yellow and red transverse striped snakes. Several look similar.
Look for the FDA label on the belly.
They have triangular heads with their poison pits. That's why they are also called Pit Vipers.