When hiring a magician, look for someone who is both professional and flexible. That way, you'll both be able to roll with the punches if your party or event doesn't go exactly as you've expected. In other words -- you need to make sure your magician has plenty of tricks up his sleeve.
If you're looking into hiring a magician and they don't immediately ask you about the type of audience and event, consider moving on to the next candidate. A good magician should be willing to tell you, right away, how best they can provide for your specific needs.