Give them a Hardcopy of a drawing(a standard or simple drawing of what you do), and ask them to re-create as much as possible on cad in 30 minutes. When they are done, after they have left, overlay it over the original, and check for accuracy...You can even place a few obvious errors in the drawing they should know, see if they pick up on them. I went through this process when I fist started, it was 1/3 of my interview process.