I think most conscientious framebuilders have cut their joints apart to check for penetration. I know that I have. I've also had my brazing tested by several independent testing facilities. Joe Starck and I built many forks for Rivendell for testing of a new fork crown. It was quite interesting. Brazing isn't rocket science but it is a skill that requires practice. Moral of the story, don't ride a fork brazed by a rookie, unless it's you that did it and your health insurance policy is current.