[anifow]

This is not a revolutionary idea. Several professional programs (law, accounting, etc.) have certification exams that do just what you are describing. It seems to require a fairly healthy, non-profit professional body in order for this to work, and you can only ever assess a fairly slim chunk of the 4-year experience, so they end up pairing in a credit-hour requirement as well.

I work in large-scale assessment, and can attest that we should hold a fairly skeptical/conservative position in the power of tests to get to the "real" skill level of prospective practitioners. All of the measures currently available are fairly weak at predicting outcomes.