I'm going to reference experience with my current partner. First of all, I don't really know. She is not really sure either. I ask, she says I dunno. We agree that we've found the cervix and we also discuss and agree that I can be either in front of or behind it. She says she feels a distinct difference and likes when it's "up here" and also when it is "back there," which I regard as the af and pf respectively (while in missionary).
When I aim for and stimulate what I believe to be her af, I raise her legs up over my shoulders and angle my penis as if it would be just under her pubic bone. The g spot is stimulated this way also, and can be incorporated with this type of thrusting, but it should be shallower. I feel a tighter area with more pressure on the end of my penis, when I reach a certain point pushing in at this angle. Somewhere past the area that should be the g spot. This is where she has a reaction too. She has said that she likes to do it, but has been hesitant because of the pressure and possibility of pain or discomfort. As I push into this area It is obvious that pleasure intensifies (all women are different here) because she makes those wonderful sounds. I notice both fornicees produce increased vaginal wetness too. The af is tighter and requires more gentleness but neither can by pounded right away. I am not very long, but when I push deep in the af initially, I hear her vagina make a slurping/sucking noise, as if I have shifted the vaginal canal or stretched it and created a vacuum. This is when she makes sexy expressions and groans in pleasure, but kind of a "hurt so good" pleasure. Shortly after that she usually requests that I pound that spot, and she squirts and or cums. In, closing I will say that I feel these spots with my penis. I use that more than her cues to judge where I am.