Instructions
On a more serious note, you didn't stipulate wether or not you are looking for a partner or want to stay single? Not to shoot down Tinuva's contribution, but those four points are not really sufficient however it is a good start (minus point 3, 1 to 4 can just purely be politeness and natural neutral response even when combined). Women who are forward will be obvious to you, so that would leave the traditional non-forward crowd. Furthermore flirting doesn't necessarily mean there is an interest, quite a few women flirt for fun/attention/boredom/whatever. There isn't a surefire way to check, however you can introduce elements that would elicit an "either or" response. Some of these methods can be a real ******* way of doing things.
This could come across as sexist/bad/insulting/whatever. My bad.
If she is really interested in you then she will get real jealous when you introduce another woman that you are well acquainted with but she doesn't know, and you provide more attention to that woman. (Trap Note! Attention seeking, controlling women will still exhibit the jealousy trade and will probably engage the other woman in order to protect their boy toy).
If you find yourself doing things or sharing time with just her (not a date) semi frequently (time spent is relatively subjective to respective individuals) on events that wouldn't naturally pair you two (as opposed to biking in the same team because you are in the same biking club) this is a mildly strong sign, especially if it is events such as cooking your normal meal (Trap Note: if you are a tool to her, you will find yourself being leveraged to do things she wants more often than you want (unless you like these sort of things). You will probably obey in order to please her or some stupidity to that effect. Stand your ground and try have things more often your way. Note, too controlling and bossy will drive away the real deal while too little will just come across as weak. You will need to find the balance that will scare away the fakes and keep the real ones, in most cases you wont find many persistent fakes.)
If you are on occasion (or more) the subject of girl talk when she speaks to her female friends (or the friend zone guys), this is another good sign. You will need to somehow find a way to extract this information from her friends.
As mentioned by Tinuva, physical contact is a key component. Try remove completely any contact you initiate and only be reactionary, as in only return physical contact after she initiated first. Exceptions are situational, such as catching her from falling (however if she does fall and injure herself, this could provide a legitimate excuse to treat her at your house). Note, some people are more physically orientated so contact on its own isn't a stable source of information. WHERE she touches and how long however, will indicate better. Hugs are a good place to start, it provides the intimacy under the guise of being friendly. How and where she hugs you will be subjective, however if she lingers when you break contact (discounting situations such as when she is sad and needs physical/emotional comfort) thats another good sign.
If she is really interested in you, she will try to integrate her life to match yours to some extent. Examples being she will suddenly find interests in your hobbies or at the very least try, hobbies she herself never had before. This rotates back to the previous point about spending time together I mentioned previously, use your discretion.
Lastly, introduce light sexual tension in the conversations (problematic if she is ultra conservative/religious/etc). This relates back to flirting, however if she is interested you will find that the lightness of the topic will escalate to mildly serious to serious-deepest-darkest-fantasies level. Anyone else will have already aborted at the mild level. (Trap Note: If she is slutty then you will likely achieve the same end result with the real deal. Good or bad, you may have scored yourself a sex buddy instead of a girlfriend).
By no means a guide or anything, nor will any of these situations combined completely or otherwise will yield the expected solution.
The ultimate fool-proof solution to this problem is simply to just tell her you want to be together if you are interested and you think she is interested. It's only awkward if you think it's awkward.
and to prevent further confusion I've been having regarding dating, I figured I'd bring my dating question's to MyBB 
