Singing, dancing, doing the occasional backflip, saying "hey, dude!" a lot and eating pizza?
No, seriously, a 'happy' or at least content turt will be bright-eyed and alert. They will pay attention to what is going on around them, and may even be quick to jump to cover or the water- it's not stress, just a healthy concern for safety and security. They will bask and feed regularly, and will explore their enclosure. Hope that helps.