Obliviousninja, having been shunned by 99% of the girls ive ever taken an interest in, I only very recently came in contact with a girl who I found out later was VERY much interested in me. But I could DEFINITELY tell a very solid difference in this girl vs uninterested girls. This girl talked to me about the most random shit, mostly whats going in her life, what she's doing. no 1 thing she always told me where she was and what she was doing at that moment, and what she plans to do. bAsically, girl tells you about herself if she's interested. And on fb she always initiated the conversations, NEVER me.
Just in general, if girl INITIATES conversation with you AND the conversation is most of the time about her life events, you can be 99% certain she is interested in you. If she just initiates conversation about a common thing between you both then obviously you cant be as sure. But if she starts going on about "i i i me me me I did this this and this and I want to do this this and this and WE SHOULD TOTALLY DO THIS TOGETHER"etc, you're in man.
Me personally being an absolute female retard, I had no idea she was interested until my brother started reading what we were writing and told me I should have stepped things up a notch ages ago, she's well interested.
There's a story from personal experience. Hope it helps man.