People who are flirting often touch themselves.
Women often play with their hair, while men touch their faces, ears and cheeks. Both sexes unconsciously adjust their clothing -- men ensure that their shirts are smooth and tucked in, while women check that their skirts are straight. These are all preening behaviors in which people make themselves look more attractive to the person they are flirting with.
When someone is flirting with you, she will say your name in conversation more often than someone who is not.
This is because she likes you and your name, and she is trying to show that she is interested by repeating it. So, if you hear your name repeated more than you would in a conversation with a bus driver or a friend, then you are probably flirting.
Men and women often try to set themselves apart from the crowd when they are flirting.
So, gestures and movements are more exaggerated and stories and anecdotes are more animated. Someone who is flirting with you basically is trying to make you like him, and he will do so by trying to put his best foot forward.
When people flirt, they subconsciously change their voice tone to match the person they are flirting with.
They don't just change their tone, either. They also change their cadence and intonation to build a connection. By mimicking your voice tone, the person you are talking to is showing that she is interested in you and wants to be like you. So, if you notice a change in someone's voice tone to match yours when you are talking to her, then she is probably flirting with you.