This is going to be hard to describe since it's more about feeling but I'll try.

When I'm handling one of our snakes it's not so much about posture because they'll draw their head back into an S for a lot of other reasons than setting up a strike. Actually most of the time I've been struck at (only one time did it connect...I'm fast lol), I don't even see it actually coming. It's just a knowing that they're about to go and I move out of range or move the snake out of my range or use something to block the strike or redirect it.

It's more about a feeling, a tenseness to them that isn't the usual way they feel to me. Like their muscles are more tense or they are more alert...darn it all, it's really hard to put into words. A lot of it comes down to getting to know a snake and it's subtle and not so subtle cues. Every one of them are different. Here's an example to illustrate....

We have two adults here, one male, one female. They are both big hissers, always have been, likely always will be. They both hiss pretty much everytime you bug them. Neither has ever tried to bite. They just complain a lot about life I guess. I have other snakes that rarely act up but if one of them gives me a big, low toned hiss....I'd back up and respect that coming out of an adult BP that doesn't normally hiss at me. That sort of hiss tends to put the hair up on your arm a bit. It's all hissing, but it's different hisses from different snakes that mean different things to me so I react differently...make sense?