I think a lot of owners never actually know their puds have FIV to be honest.
I think the majority of us owners on here have adopted cats from rescues, where they are often tested when they are picked up, particularly if they are strays, and almost certainly if they are entire males, as these are top of the list in the fighting pile so more likely to have been exposed to it (its passed on mainly through biting, saliva entering the bloodstream).
Most owners wouldn't normally test for FIV unless the animal is ill, which is often when it has progressed a little for symptoms to come out.