First of all, RTFM! Oh, sorry... READ THE ****IN MATERIAL! Read the DJ Bible, read DYD, read MM, read, read, read...
Yet, those places/books are the last ones where you can find the answer. ( I'm tellin g people since long time ago to use their brains instead of sosuave ) You will have to deal with girls for a long time and gradually you will be able to detect when a girl is playing games, or even better you'll be able to notice when she's throwing sh!t-tests on you.
But if you really want ONE short answer, then I will tell u this. It might be wrong, however:
Most of the times when girls ask for favors, it's a sh!t-test. She tries to figure out, throught those tests, whether you're a man or not. However, sh!t-tests can be camouflaged that well, that you'll understand them only when you fail. But donjt get it all wrong because sometimes girls ask for favors because they really need smth. ("Girls are people, even though they have puss!es" - Fingers) In that case you should be sweet and do what they are asking, but never forgeting to set a price or a condition for your favor.
Other way to realize whether a girl is playing with you is to pay attention to IOI's (Indication Of Interest) , but they can be faked as well, and i got burned so many times .
Usually when you're someone worth, but fail to create attraction on her, she wants to keep you so she can increase her social status and brag to her friends about how she's playing with you.
Back on my completelly AFC days, a girl tricked me really nice. She played the push pull technique on me. (yeah, i know it's supposed to the other way) One day she would act as if she's trying to **** me, and the other day when i would finally get the balls to do smth she would completely ignore me!
There is a lot written much better on this topic so i think you should consider reading the material i told you about.