As mentioned, in California (and, I believe, some other States) pretty much all transfers must go through an FFL. I'm sure that there are illegal, fully private transfers, but I'm not sure that anyone knows how many.
I suspect that the level of voluntary compliance is quite high. It's not all that inconvenient, and most folks want to avoid the potential complications of participating in an illegal, felonious act. It will take two people willing to accept the risk to do an illegal transfer.
How to catch people is law enforcement's problem. But I bet a few get caught by bragging about it on the Internet or to their buddies at the LGS or range (perhaps withing earshot of an LEO).
"It is long been a principle of ours that one is no more armed because he has possession of a firearm than he is a musician because he owns a piano. There is no point in having a gun if you are not capable of using it skillfully." -- Jeff Cooper