Well, corns can hiss. That's one way you can tell they aren't happy. They can't smile or grin, and they definitely can't wink, since they don't have eyelids.
If they're unhappy, they might hiss, hide all the time (this could also just be b/c they're digesting or getting ready to shed), try to bite you when you go to pick them up, or other things like that.
You just have to observe your snake and get to know how he acts when he's happy, I guess. A well fed snake that has enough room, enough water, a good place to hide, and is warm enough is probably fairly happy.
K
PS: Mine seem to enjoy tunnelling, so if you can provide a substrate that your snake can tunnel through, like aspen shavings, he might have a bit of fun.