How do i know if a girl likes me?

So theres this girl i met by one of my good friends. They like eachother but ever since i met her i have been hanging out with her alot and she is being really flirty.Not to brag but i do usually get the girls over him but im just curious. we text alot and some night we have 2hr phone calls. i skype her alot but im always the one texting her first.