A happy cat has a healthy appetite and eats regularly; keeps clean and shows a well-kept fur coat by grooming itself and others; loves sleeping, and enjoys social cuddles with other pets and you [the E.G.S scheme], plus it shows playful behavior, and uses its litter box properly!
More specific happy-affectionate signs are showing trust by revealing its belly to you; generally likes to hang out with you and follow you from room to room. It will give you 'head-butts', rub against your legs and arms, show off their butt to you, purr and meow back when you talk to them. A happy loving cat will knead on you before sitting on you and blow you kisses (slow blinking), will nibble you ( the cat's version of love bites), and bring you 'claw-ful' presents like tree leaves or even dead mice.