Unfortunately, I can't really give you the best answer because I've never been on the other end of the transaction.
However, I think many of the same rules still apply. The difference is that since you're the person potentially paying the other party, you hold all the cards. You can basically find out everything about the potential hire by asking; they know if they don't play nice, they won't get the job. Asking lots of pertinent questions is enough to scare away many would-be hires who are less-than-serious about getting the job done right.
Look for developers who have a solid portfolio. They don't have to have dozens of sites under their belt, but it's important that they have a few. Ask for references, and follow up on them. A good developer has good relationships with his clients, and one of the best ways to measure someone's performance is by talking to the people they've worked for in the past.
Like I said, I've never been on the hiring end of this equation, so the above is all the advice I have. Use your gut -- if something doesn't feel right about someone, move on to the next.