She likes you and that other guy and her boyfriend but does not know who she likes best.
The most appropriate way is simply to ask her.
If u like a girl who already has a boyfriend but you dont know if she likes you or know,the first thing you have to do is that you will have to make friendship with that girl and if she is already your friend,then increase your friendship and one day confidently ask her that what does she feel about you..
Then she has to choose you or her boyfriend.
a way to know if he likes another girl is if he keeps looking at her and smiling also if he talks about her with his friends.
I would confront the girl or warn your boyfriend
um the way she looks at u or somethin
if i where you i would tell the girl that likes that boy that he has a girlfriend.that is what i would do.
Honestly, if ur a good boyfriend. and you honestly know it and knows that she really likes you too, then ALWAYS is my answer.
She textes u everyday & and starts asking about ur life
Normally she will flirt with you, and want to be around you a lot.
you slap him across the face. I hope that helps :D