Posture is everything with lovebirds. How they hold their head and the state of rigidity their body is in will tell you all you need to know about how your bird is feeling and if she wants to be handled or left alone.
Your bird's posture can tell you most about her mood. If her body is relaxed, but her head and body are up, she is happy and content. If her head and body are up and alert, but her body is stiff and her feathers are flared, she is probably feeling territorial. When she is crouching with her head tilted down toward you, she is asking to be scratched or petted. This can also mean she is begging for attention from a mate. Some birds, prior to defecating, may take several steps backward, crouch, and lift their tail. This is a harmless posture that should not worry you.
If a bird is crouching with her head down, eyes pinning, flared tail feathers, ruffled feathers and a rigid body, weaving from side to side, she is giving a warning. This usually means to back away, because she is ready to bite if provoked. She may also use hissing and a raised crest as additional clues that she is not happy, and she needs time to cool off before being handled. If she is standing with her beak open in a crouched position, especially when combined with hissing, your bird definitely needs space, as this almost always means she is prepared to bite.
Unlike humans, birds have the ability to control their irises, which enables them to communicate if they are very interested in something, excited, frightened, angry or aggressive. At certain times, your bird's pupils will enlarge or shrink, which is called "flashing" or "pinning." This method of a lovebird's communication is not sufficient alone, so look at your bird's other body language before deciding how to handle your bird.
Vocalizations are another way of knowing how your lovebird is feeling and what he is thinking. When combined with an understanding of body language, it is a great way to read your bird's emotions. There are very few unhappy sounds a lovebird makes, one of which is growling. Growling is a sign of extreme annoyance and is a sure sign to back off. Purring can also be a sign of annoyance, but might mean your lovebird is happy and content, so carefully watch the body language that accompanies this sound. Lovebirds also make many happy sounds that can indicate they are in a good mood. Singing, talking and whistling are indications of a happy bird, while clicking her tongue means that your lovebird wants your attention, or she might be happily entertaining herself.