Before hiring look at the portfolio of the developer. Ask detail questions about what type of apps have they developed till date. Referring to their work, you will understand their proficiency, creativity and what type of iPhone apps they can build. You can even ask them to provide the link of the app developed to find out the utility on iPhone devices.
Get information regarding their technical expertise and knowledge about iOS SDK and other tools and technologies like JAVA, Ajax, Open GL ES, Objective C, MYSQL, HTML5, CSS 3, etc. For the skill assessment for software engineer you can use Employee Assessment Tools & Recruitment Assessment Tools of Wild Noodle.
Image Source: Google
Whenever possible ask for Client Testimonial, he or she has worked with. This will give you clear idea of their credentials. The clients may also help clear your doubts that you may have regarding developers' expertise. This will help you to conclude that whether the developer is good at work and will able to provide high quality services or not.
It is also vital to find someone with good communication skills. If the developer lacks communication skills, he/she will not be able to understand your requirements properly or will not be able to explain their views clearly. A developer should possess good listening skills too.