There's no way to know exactly during the fuck that does it. I had my hopes, and the sex was incredible because of them, but I had been disappointed before. I just went back to my daily life and waited. Sure enough, the fuck flu was what told me. I was both scared and excited. A test confirmed it, and I literally did get hard in the office when they told me the results. I started to feel a little bit freaked later (like, what have I done??) but that only lasted briefly before I felt happy again and at peace with my new status.
I will say that I am very glad I got it the natural way, through sex, as opposed to a blood slam. I know there are guys who are in a hurry to get it over with (I more than understand that!!) but the beauty of knowing that this life is inside you because a man, well, made love to you -- I can't describe how awesome that is.