With multiple cats in our home, we learned about ears and tails pretty quickly. I do remember the first time our first cat sat in front of the window with his teeth chattering and his tail twitching - we thought he was possessed or something!
Of course, now we know he was just interested, and in "get the prey" mode.
Tails puff during play as well as in true aggression - so it's important to put it all together with the posture, the ears - and the context.
I`m new to cats but it did`nt seem to be all that complicated for me to figure out Alley`s body language. Once I began quietly watching my Alley cat during her daily routine of napping,eating,sleeping,resting,watching out the windows and also how she acts when trying to get your attention or service--you`ll start associating your cat`s body language with her activity and her mood. Watch the tail and how it moves when at rest and at play.Body posture and tension also will change with your cat`s particular moods and frame of mind. When my cat is excited and in full-play mode her pupils dilate to almost the size of her whole eye!! Classic "cute kitty" face! When really frisky(or surprised) Alley gets the "bushy" tail and sort of does the Halloween Cat w/arched back and bouncing sideways.Funny!
Bet you that cat already knows your body language! My girl also is becoming more vocal as she matures;she makes chirps and cooing sounds for me to make the faucet drip each morning(likes to slap it and hear drain sounds).I believe the more you talk with/to your cat--the more she will interact with you.Just my $0.02 and some observations.