This is a skill that is critical to your success with your parrot. An inability to ready body language accurately can result in serious behavior problems, the most common of which are aggression and excessive fear.
The need for this was not recognized until the past decade or so, however. A lot of information was published prior to that period about the need to be dominant in our parrot/human relationships. If you're going to dominate your parrot in order to get the behavior you want, why bother to read body language? This attitude, thankfully, has now been recognized as both unethical and ineffective.
We want to have mutually trusting relationships with our parrots. I want to have the assurance that I'm not going to be bitten when I interact with a parrot and I want my parrot to understand that I won't force him to do anything.
In my blog post "Essential Guide to Communicating with Parrots," I make the point that any trusting relationship depends upon good communication. Good communication with parrots depends upon two things: (1) the ability to read the parrot's body language accurately, and (2) the use of positive reinforcement to motivate our parrot into performing desired behaviors.
The examples below are few at present, but I will be adding to this library as I go on and find images that represent common emotional states in our parrots.