If a girls best friend, keep saying she likes you...is it usually true?

It would seem in this situation that she could be playing you along a bit. I mean, if you don't spend that much time with this girl, then how can you be together seriously? I would suggest addressing her face to face and questioning her on this. If she is just having a laugh, then she'll either deny it or awkwardly tell you how she feels. If she does fancy you, and does want to get together, she might show aa few signs. If you do want to get together, and she does show signs, then chase her.