Here are several:

1. What other sites did you develop? Ask for domains and the actual parts he/she worked on (i.e. design only, functionality of X, Y or Z)

2. What's your background? You're looking for someone with real experience, not someone with just a degree. You're also trying to figure out if he/she "thinks" on your level, because if they don't, you might not get what you want.

3. What other tools have you worked with? A good developer will have experience outside of a particular shopping cart - even if he only works with one cart. Those other tools would be MySQL, WordPress and the likes. This lets you know he can INTEGRATE things, rather than just build on a template.

4. Ask him if he's worked with any other professional people in related fields. For example, if you are also hiring an SEO strategist, he needs to be able to incorporate tags and such. If you're hiring a graphic designer, he needs to be able to incorporate those components.

These, of course, are just a few questions. The type of project and preferred language will call for many more.