Mikah is always running around and jumping up in the fake bamboo tree we have.. when i walk in my room she purrs and meows and has such a positive attitude. she's always coming for more of my love.. you can tell if they're not happy by body language and also sometimes there pupils.. i've heard the pupils will be larger if they're stressed.. but it can also happen if it's dark :P

also shedding.. if you have a cat that normaly sheds its not a big deal.. but if you notice your kitty only sheds when it seems angry (and maybe it's a shorthaired cat so you know there shouldn't be much shedding) then seeing if hair is coming up when you pet them can tell you if they're happy or not

also (sorry for blabbing :P) if your kitty rolls on his/her back and exposes their belly that is a huge sign of trusting you and over all being comfortable.. it's them telling you that they're happy in the enviorment, enough so to let there belly out :P