its kind of hard to put into words. alot of it is subconscious pattern recognition. The more we do a certain activity the more our subconscious brain recognize patterns. Some things that i can explain would be eye contact, facial expression, body language, tonality and proximity. Ie, when a girl is looking me in the eyes and standing somewhat close to me and a smile theres a high likely that shes into me and conversely when a girl is looking away and keeping her distance with her arms crossed, chances are shes not that into me. Theres a lot of grain in between which comes with experience