I'd say 80% of the time she's telling you the truth about the way she feels (although that doesn't mean it won't be true in 10minutes)
I always treat it as a test, but sort of an experiment to see if I turn her. The odd one I have I told her I was not interested in her now I'd got to know her (just a spiteful wan ker sometimes)
I think you learn more about your game in these situations than in any other, but in terms of a result it's unproductive.