Healthy, well-socialized kittens play nonstop and simply don't know any better than to be happy. Playing is a terrific indication of happiness.
Engaged and Confident
A happy cat shows interest in its environment. It may be the "supervisor cat" with a paw into everything new around the house. Or it may simply watch calmly from a distance.
1. Notice Her Tail
The position of a cat’s tail will usually give away a lot about their mood. If they approach you with their tail up in the air and a slight curve at the tip, this shows that they’re happy to see you.
Vocal kitties tell you all about it and engage in long drawn out conversations. Generally, the higher-pitched sounds are "happy" while lower-pitched can have a bit of frustration and be more demanding.
Cats may sleep more when sick or depressed, but their sleep location preferences can be an indication of happiness.
Cats who feel good keep themselves well-groomed. Poor grooming is a sign of unhappiness, illness, or injury, so when your kitty looks spiffy, that's a positive indication it feels good.
Purring and kneading
The gentle rumble of a cat’s purr usually signals that they are feeling relaxed and happy, and may also be accompanied by kneading.
Happy cats have healthy appetites. Oh, they may bluff and train owners into providing special treats or food.