(Putting on Nomex suit)...
Actually, there is no sure way to tell. Women will play this game of attraction for so many reasons that you can't count them all. Look at Anna Nicole Smith. Does anyone in a million years think she was sexually attracted to that geezer? Or the women who hang out with Hef at the playboy mansion?
Most men are basic creatures. We like sex. If we can get sex, that is great. If we can find love with sex, that is even better (provided we are mature enough to handle love, that is). Women understand that they hold all the cards in this society when it comes to sex. They use it like a weapon, to control the men around them. The challenge for most men is to not confuse sex with love. Most women NEVER confuse sex with love. Always remember that.
FOR MEN NOT READY TO FALL IN LOVE AND GET MARRIED: It really doesn't matter if she is sexually attracted to you or not, what matters is: will she sleep with you. If she sleeps with you, you have a happy ending. Does it really matter if she is sexually attracted to you if she crawls into bed with you?
My philosophy is: just take her out on a date, and ask her! If she says yes, then close the deal. If she says no, then all you have lost is the cost of a date. Either way she may be lying, but it does not matter. Unless you are totally in love with her and do not want her half of the relationship to be a sham...
FOR MEN WHO HAVE FALLEN IN LOVE, AND WANT TO GET MARRIED: If she is the one who initiates sex more often than you, even after the engagement/marriage, then you may have a winner. But there is no way to be absolutely sure.
Of course, there are women who are going to say they are honest about this...but they are just doing what they can to perpetuate the lie.
Use all the flamethrowers you want.