I've been doing web development for about 15 years now and have seen some great methods for finding the right candidate. One of my favourites for finding a solid programmer goes like this:
Start by asking three questions. One simple, one medium difficulty, and one impossible question. The idea is to see how they think. If you can get them to work it out in front of you this is a great way to see how they will problem solve issues. The first two they should be able to answer, and the last they should be able to figure out they don't have enough details to do, and why. If they can't figure out that it's impossible, you can always let them know, but see if they can figure out why.
If you have to code in a specific language, ask questions about that specific language. Keep them relevant to your project. Same with if you're going to use some pre-made CMS, or what company you are planning on going with for payment processing. Find out if they have experience working with those things. This isn't always important, but can speed up the process. More important if your project is time sensitive. If they suggest using something else, find out why.
All of these things should make someone stand out for you. Sometimes it just comes down to a gut feeling.