I agree with brian. It will require some good project management on your part, but if you have a good design you should be able to break off some smaller tasks and give them to prospectives on a trial basis. Spec the tasks out fully and make it clear what you want. For example, code it and provide tests to demonstrate it works.

You didn't mention whether it was hourly or piece-work. If hourly, you get a feel for how much time it took them when they report back. Piece-work is where you set an estimated hour value or dollar value for a feature and the coder does the work for that amount. This can be tough when estimating, and you may need to be prepared to be fair if a task ends up being bigger than expected.

Having many smaller, defined tasks also allows you to pursue a few coders. If one or two don't work out, you still have some people working. When you get to the final integration, which can be the tough part, you'll know who you are getting the best work from.

Also, if you know what frameworks you are going to use to build it (what modules) that can help. Sometimes the module authors themselves are contractors, so you can literally get the expert in a particular module or technology working on your project. This has the advantage that they can add features to their module if you really need them.