A snake is primarily muscle - so when the snake is uneasy you can see and feel the muscles tense up and when it's relaxed you can see it relax. If you are holding a snake that doesn't want to be held -- it will be all tensed up and hard and likely try to flee or poop on you -- once it relaxes it will be a soft and mushy snake that wants nothing more than to curl up in your warm lap.

As for babies -- some are nice enough to huff and puff and get in the strike pose prior to striking - easy enough to read. And others just randomly launch out and bite at anything they can -- they tend to outgrow this little maneuver.