Really, you can't know for sure unless they mention it or I suppose others mention it. It's all in the luck of the draw when asking people out even if 2/3 girls are taken.
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"The less I know about other people's affairs, the happier I am. I'm not interested in caring about people. I once worked with a guy for three years and never learned his name. The best friend I ever had. We still never talk sometimes."