There's being relaxed and still and there's being tense, guarded and frozen in place still. Just like there's interested exploration of an enclosure and then there's stressed and frantic slithering about.

As you get to know your own snake and develop your ability to observe and see trends, you'll begin to instinctively know what's going on to a reasonable degree. Just be observant, be tuned in and learn to read those subtle and sometimes not so subtle signs the snake is giving you.