Instead of constantly being frustrated that you just can't read your cat's mind, consider the possibility of reading her tail motions. Cats are extremely communicative creatures when it comes to using body language. One of the most expressive parts of a cat's body is her swishing tail, surprisingly enough.
If your precious pet's tail is lightly and casually switching back and forth, she may be in a thoughtful mode. Perhaps she's deciding exactly what to do at the moment. Maybe she wants to go eat in the kitchen, but she also wants to take a nice, long and leisurely nap, too. This cat is feeling uncertain and is mulling things over in her adorable feline head.
If you are giving your little one's back a rub while she's lounging around on the couch, her tail may start flicking and switching back and forth as a way to show you that she's kind of annoyed. She's had enough of the petting for now -- "Too much stimulation, back off, thank you very much." At this point, your cat is simply irked, although not to the point of anger. However, if you keep on petting her, she may quickly escalate to that point, so give the fluffy princess what she wants.
When a cat abruptly and almost violently begins switching -- or perhaps even thrashing -- her tail around, then she's in full blown anger mode. In fact, she's so angry at the moment that she may even get aggressive. Back away from this furious cat. If a cat is this angry, she may even bare her claws or teeth, so be careful and stay away. Let the cutie cool off and regain her usual composure.
When only the tip of a cat's tail is swishing, it could indicate that she's feeling unsure of herself at the moment, and perhaps even a little bit nervous. Maybe the cat is considering jumping onto a very high surface and isn't certain that she can make it. This tail positioning usually means your cat just isn't feeling like her usual confident and brave self at the moment.