There are a lot of exceptions. Most of them, as it happens.

Vipers have that "neck" and a wider head than their bodies. Elapids typically don't, and can be extremely venomous. In fact, the most deadly venomous snakes in the world are elapids including cobras, taipans, and black mambas.

Tl;dr: Rarely wise to step on snek.

If red touches black, you're ok, Jack.

If red touches yellow, you're a dead fellow.

This aphorism only works in North America, but it is a pretty reliable way to determine the difference between a coral snake... and a milk snake, also many other kinds of similar looking, non venomous snakes.

You may note that the milk snake has a bit more of a defined neck, head vs body seperation, than the venomous coral snake... which ... would mean if you followed your rule, you may end up a dead fellow.

...

Now... many, venomous snakes make some kind of an alert sound, a hiss or rattle or someother bodily mechanism of saying 'back the fuck up'.

But not all of them.

... and a great many venomous snakes... well they hide in the shrubbery and tall grasses, meaning you can easily accidentally come upon one if you're moving through brushland, or a wetland...

You're right that you should never intentionally sneak up on a snake... but... it is usually more like accidentally happen to be too close to one, hear the alert sound, waaay too close to be comfortable... and then you fucking freeze, try to figure out where exactly it is by your ears alone, and then very, very slowly back away untill you can't hear the rattle anymore.

At least thats what I did when that happened to me, and I lived, to insufferably recount the tale as I am now, lol.

There's... only one kind of rattlesnake in Eastern Washington state.

And it is venomous.

Say hello to my missed connection:

But hey, your 'does it have a neck' rule works for this one!

Too bad I never saw it, at all... its got pretty good camoflauge for the one to two feet tall grasses and shrubs it resides in.

Snake bites are of course, overall, a very uncommon thing for most people to be worried about... but if you are regularly involved in some kind of outdoor activity, or just kinda live out in the sticks, or are renting an AirBnB out there... you should probably familiarize yourself with the local wildlife.

...

As a final note, I am not any kind of snake expert at all.

But I do know that if you are, then the actual word for that is... Herpetologist. Expert in Herpetology.

Consult your local Herpetologist before you derp around in the badlands, lol.

...

EDIT: Final addendum: Female snakes often tend be considerably more pissy, apt to warn and stike, when they are in heat.

that rule has so many exceptions that it's better to assume they are all dangerous