So what are your main options for making headway - or at least figuring out if she's actually interested or she's just a big flirt - with a girl you think is into you, maybe, but who never does anything with you?
If the girl sending you mixed signals is in Category 1 - she's genuinely interested in you, and just playing a little coy - you won't have to worry too much about "figuring her out," because as soon as you start taking steps to progress your relationship with her, she'll come right along with you.
Once I had enough confidence and experience with women that I was accustomed to acting quickly on signs of interest, my own dynamic with women giving mixed signals changed quite dramatically.