Literally sit them down in front of a computer, give them a text editor (just to make things even worse, give them Notepad) and tell them to belt out a complete, 100% valid, semantic and warning-free Web page.
For graphics, find out what graphics program they use, then sit them down and have them create the layout for your site.
Keep in mind this is the "final" stage of the selection process.
What about before then?
Well, you have to check their business references, before you even do anything, of course. Also take a look at their portfolio. If the sites they have in their portfolios appear to be visually stunning, but the HTML doesn't even validate, then thank them for their time and show them the door. Same thing if their code is not semantic, or even formatted properly.
If the HTML does validate, check their CSS for errors and warnings. Again, if any show up, thank them for their time and show them the door.
Yeah, I'm hard. I have to be. These are the standards I set myself to. Theyr'e also the standards I think we ALL should aspire to. The longer we let sloppy code exist on the Web, the harder it's going to be for us to improve our skills, while also showing those who follow in our footsteps that such bad practices are not only acceptable, but the proper way to do things.