Conceivably, a system of tests or certifications could be used as a reliable signal of competence. Just make a candidate go through a battery of exams. The issue is that firms most likely wouldn't have any interest in doing this and any independent organization doing this would be tempted to change into the business of selling certificates, which would make the certificates themselves worthless.

Which isn't to say that the issue is unsolvable. But it still needs solving. I don't think anyone has paid much attention to it.