How does one know?

Ok so how do you know if a guy at school/work etc is developing a crush? And im not talking he'll look at you more, he'll try and talk to you. I'm talking like you normally don't talk to this man and you have suspicions that he's crushing and you might slightly be crushing but don't want to overthink it. How does one find out this information when he's not part of your friend circle?
How does one know?
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