I can't tell you how, but I've never yet picked up a single bulldog that I didn't get to catch. Now, I have culled all of them except the first one and my current on for some reason or another but never yet picked one that wouldn't catch.
Best "advice" I guess, would be don't look for a big muscled up fat head pit of any color. I got a blue pit one time, she would catch, but I culled her quicker than any other for a multitude of reasons, my best luck has always been with pits who are built like curs with a pit head.
I'm with you T Bob....never had one that wouldn't catch.