When you are complimented by people you yourself find attractive. When you get a relatively large amount of attention from the opposite/same sex. When people tend to be drawn towards you regardless of what you are doing.
People compliment you and members of the opposite gender...or hell the same sex lol...check you out.
I used to have low self esteem and I thought that I wasn't all that attractive yet people kept complimenting me and guys kept checking me out. So it was like "Well if you're really that ugly, then people wouldn't say you're pretty all the time and guys wouldn't be hitting on you"
as a hypnotherapist, I study body language to a degree so here's a little clue. When we first see someone, we unconsciously give off signals that reflect how we feel. if a stranger looks at you and her eyebrows even slightly go up in the middle, she's attracted to you. I probably shouldn't be telling you this because what you most likely need right now is not to obsess about "did she look at me like that? what about her? and her?". what you need is to fully realize that it doesn't matter because everyone likes something different and having the confidence to meet many new people gives you the opportunity to meet the girls who will be attracted to you