Well, telling whether someone likes you can be a little difficult. Now, if you’ve known this girl for a while it can be both easier and more difficult. Having known her, you’ve probably seen the way she acts around people she’s dated or wants to date, so you can use those interactions to judge how she’s feeling towards you.
However, in my experience sometimes people are scared to let others know that they like them, especially when there’s a pre-existing relationship. They get scared that trying to push for more will ruin what they have.
That’s kinda what happens with me sometimes. For example, I was kinda infatuated with one of my teammates, but it took a near-death experience for the two of us before either of us could admit to it. Sometimes a fear of rejection is stronger than the feeling of longing.
The best way to tell if she likes you back is to straight up ask her out. If she doesn’t like you that way, she’ll tell you.
But if you’re looking for a subtler approach, my advice is to watch her carefully when she’s around you to see if you can pick up any of those little flirty things that you’ve seen her use on people she’s dated while you’ve known her. If you don’t feel you have a grasp on those, then watch for differences in the way she treats you versus people you know she has no interest in.
But seriously. Just ask her out.