Look up body language on the internet, there are heaps of articles about it. Human beings are basically primates. We indulge in lots of display behaviour when attracted to someone.
For example a girl may play with her hair, or turn sideways to you, or drop one shoulder, or blush, or make eye contact and then let her gaze slide away from yours, or play with the stem of her wine glass. Two or three of these signs need to be taken together.
It’s usually unconscious which makes it very accurate.