I think it depends on what capacity this person will be working with/for you in the future. If you are hiring him/her in a professional setting, test them in a way that gives them as realistic an outlook as possible - you should definitely create your own test.
For instance, if your consultancy builds static sites from PNG designs, have him/her turn an image into a functional web page by any means necessary. If they lean on Dreamweaver or another tool (assuming you're looking for hand-written ability) you'll find out pretty quickly.
If your aim is to teach and evaluate students, your end goals may be more in tune with encouraging best practices rather than a fully-functional product. If that's what you're going for, have them use the W3C validators for XHTML and CSS, as well as something to verify scripts (Chrome console, Firebug, etc.)