For any iOS/iPhone/iPad developer, look at the apps that they have already developed and preferably are in the App store. See what features these apps include, how polished they look and feel. Ask what parts the developer did themselves.
If you want a native app, make sure they know Objective C and Xcode well, and didn't just use a bridge, such as PhoneGap or the flash converter, etc.
Outside that, you might also want to evaluate their communication skills. No use hiring someone who doesn't understand you or your product's concept.