how can i tell if a girl is DTF?

hahaha I've never heard of the acronym before but I love it. One thing you could try is bluntly asking if she wants to F, where it could be taken as a funny joke. If she just laughs uncomfortably it means no. Any sarcastic or relatively promiscuous response probably means yes.

Yes. Go to a hooker and pay her. Because it's obvious that is all you want from a girl, and it's obvious to them, too, which is why you get shut down. If you want more than a quick lay, then act like it, and don't try to lay them on the first alone time (unless they clearly signal that is what they want)...instead hang out with them, go to the movies, skating, whatever until you become close enough to develop an emotional attachment. Cuz with most girls it's about emotion more than animal passion.

Change your strategy - common sense. Start dating a girl and treating her like a special person. You will find it far more enjoyable to get to know someone before you are intimate.