Yep, that behaviour when being picked up/handled, sounds perfectly normal.
A major sign of stress is if they stop eating. Sometimes if you scare them, they rattle their tails and make a rattlesnake noise as a warning to go away. Occasionally they can hiss, but it really sounds more like a little puff and not at all the frightening sound it ought to be!
Other than that, you really have to get to know the snake over the coming months. Once you understand what's "normal" for it, you'll be able to spot problems brewing.
I once managed to tell that one of mine was sickening for something, two weeks before she actually started exhibiting any symptoms. Her muscle tone felt wrong when I picked her up - normally she's quite strong and muscular, but back then she felt a bit... well, I can't think of a word other than "flabby". She started lying around the viv out in the open, when usually she prefers to stay hidden.
Once you get to know your Corn, the slightest changes will start to become obvious and you'll know when they're getting ill or stressed.