Not sure what you’re looking for – it’s nearly impossible to do a universal assessment. Even license exams aren’t that thorough. You’d be asking to test: horticulture knowledge, grading, drainage, materials, detailing, specifications, software, office standards, writing, codes, erosion sediment control, design, drawing, modeling, etc, etc, etc.
Best way most faculty keep track is by staying in touch with firms that hire their alumni – most faculty know which students are good in their eyes, asking seasoned professionals helps figure out what the alumni learned in school and what they’re missing.