Actually it is quite simple to gauge the strengths and weaknesses of a cad operator. Two things, first sit down with that person and ask questions followed by a few show me's. These questions should be very specific and on topic such as:
Do you use osnaps? Do you think about what layer you are on prior to adding entities? Do you know what the company standard is regarding xyz (and make it something simple, later ask a harder one)? What is their understanding level of the commands, do they know what options are available to them (or do they simply hit a button and things magically happen)? Do they understand the importance of the need for consistency in drawings? Are they by nature, a renegade/the rules don't apply to me kind of person?
Ask what you need to, and mix some easy and hard questions (meanwhile assuring the person that it is ok for them to not be able to respond to all your questions since even an expert would have difficulties).
Secondly, look at some of the drawings that they have worked on recently. But look purely from a critical drafters perspective. Is this the kind of drawing You would want to work on after someone else? Check for the company standards, check for intersections, correct layers(Layer isolate works great for this), proper text, etc...
