How can you tell if he is in love with a girl through body language?

I read some stuff about body language, but I want to hear personally from people. What do you do when you are in love with a girl or a guy? Give me some details on body language and how to tell if the person really likes you? or is in love with you? What do you do? Thank you! :))

By the way, I am in Psychology, but I am not at that point yet of learning about body language etc. Until then I am just curious about it, hehe.
How can you tell if he is in love with a girl through body language?
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