On one trip to Cozumel I booked one of the smaller six-pack boats. We happened to be staying right beside the marina where he docked his boat so we met him the next morning to start our diving. He never asked to see a card or anything. So while we were doing our surface interval between dives I asked him why he didn't ask for us to show it.
So he asked me how much I had spent on my equipment and I told him. He asked me how much the trip was costing me and I told him. So he then told me that no one would spend a couple of thousand on equipment, $1500 or so on a trip, and not spend the few hundred it takes to get certified. Made sense to me.
And to this day I have not been with a better operation and I've never seen a better diver in the water than him.