The biggest mistake this question makes is assuming that flirtation itself is a medium used to communicate interest.
Flirting is, and has been throughout history, more of a recreational mind-sport, and a way to give your fellow human beings the occasional ego boost to get them through their day. Everyone loves feeling attractive, and merely pointing out attractiveness should never be taken for more than that.
That all said, flirtation is often an opening indicator of interest in the US these days, primarily due to the proportionately higher chances that it will be mistaken for direct attraction… There are fewer and fewer playful flirters on the social scene these days – it's turning into a very rare breed indeed.
My overall advice on dealing with flirtation is simple: get thee to a flirtery! If you can't find an intro to flirting class nearby (there are tons of them in the San Francisco bay area, but I imagine it's not as common in, say, Idaho), or spend a few solid months living in the cultured deep South or Western Europe, then I recommend trying the following exercise:
For the next 30 days, every day, when you notice (not just see, but notice) someone, compliment them. Don't go overboard, or have too much intent for it to go further. Even just saying that someone has cool shoes, or a interesting hairstyle/accent/handbag/iPhone app is enough.
I recommend striving for at least 5 per day… The first week, feel free to include friends and family, but focus on strangers only from the second week onwards.
By the end of the month, you should be able to confidenty compliment everyone you see, even hundreds of people per day if need be, and your compliments will be seen as genuine. In truth, they probably will be, because you'll be far more observant of the world around you than you were before, and a lot less desperate for a particular response to your most casual of flirtations.
With that amount of skill and experience, it should be very easy for you to "flirt back" in response to an opening flirtation, and easily gauge whether you are merely playing a game of "compliment badminton", or if she's genuinely interested in taking things to the "Starbucks awkward coffee date" level with you. 😉
If you can't wait a full month, and you can check your pride, assumptions, and violent heterosexual rage, you can gain a similar amount of experience in around a week if you focus your efforts in local gay bars. 🙂