How to know it ?
Try this
EYE CONTACTIf you see her and she sees you into your eyes for
8 seconds
80 % chance she/he has interest with youTRY THAT !

perhaps ! Think that there is a girl in the university which follows lessons with me and everytime that I speak with her, she looks me in my eyes more than 8 second, maybe 30 second, it is total absurd ! Do you think that she is affected to me?