What would be good is a test to evaluate an employees knowledge and capabilities to better direct the employees training and development.
Job Interview would probably be easiest solution. By someone with Subject Matter expertise.
The first shop I worked in had a test at the interview. It would consist of simple math add subtract multiply divide. A little bit of geometry trigonometry and some of the formula to calculate speeds and feeds. Questions like how much does one degree rise in one inch.
I suspect you would need a battery of tests to cover a myriad of skills. If you had a documented training system that had an integrated database. Having each employee (and future employees) take the tests, not only would you have a training record that will satisfy most Quality Systems, but you could identify voids or weaknesses in individual people. In addition to specific skill-based knowledge ( how to run a specific CNC machine or qualified to setup or qualified to inspect) you could include safety documentation, HR documentation (drugs, first-aid, ethics, Quality system procedures, etc)
Check into Lean MFG. Have the employees do a Kaizen event at each station and give a presentation. I use there findings and documentation to cover the whole shop. One might need a set-up sheet as well. This allows you to copy and paste all questions for new employee interview as well as employees that are in training.