You can tell from a cat's tail how happy it is because you've already seen this a million times. A cat will follow you into the kitchen with it's tail pointing straight in the air when it thinks it's getting a treat (maybe a cheezburger?). And it usually walks out with that "'s all good" expression on its tail.
The happy cat tail is pointed up like the angry cat tail, except the tail is deliberately dainty. A cat with a big fluffy tail can be misunderstood by other cats that aren't used to it -- it looks ready to fight all of the time!
A cat trying to decide if it likes you might keep its tail slightly lower, or curved. The cat is trying to decide if it likes you enough to put its tail straight in the air, or if it should tuck it down as a non-aggressive act of submission.
A cat flicking its tail usually means that the cat is slightly annoyed. It can indicate interest, but you'll be able to tell by its ears. If the cat is actually happy, its ears will be up and alert. Otherwise, big flicking motions show that the cat is annoyed.
A cat with its tail pointing up but puffed out means it wants your attention, so that you know that it wants to claw your face off. It's the "don't say I didn't warn you" look. Don't say we didn't warn you.