1. Ask your friends.
Your friends are your friends for a reason- they are similar to you. If they had a good experience with a photographer it is worth scheduling a meeting with them as well. There are so many amazing photographers out there that rely on word of mouth. I know some very, very talented photographers that suck at marketing that you may not stumble upon on Google but who will do an amazing job at your wedding. People do business with people like them so your friends' photographer will probably be stoked to do business with another cool person like yourself.
2. Look at their website.
You can't know if someone will be a good photographer if you don't look at their work! Look at their editing style. Is it consistent or does it look like they used a bunch of random filters? What is their style? Natural? Lifestyle? Documentary? Photojournalistic? Traditional? Look at their clients. Do their clients remind you of you? Read their "about me" section and see if they seem like someone you could vibe with. They will be all up in your business on your wedding day so make sure they seem like someone you feel comfortable with. Also check other sites like Google, Wedding Wire and The Knot to see what past clients are saying about them. Have they been published? Have they won any awards? What is their experience? Their website should be as professional as they are.
3. Meet them in person.
Once you settle on a few good choices, set up a consultation with them. People are simply different in real life. You will get a real feel for someone if you meet them in person. Ask them to show you a full wedding album. Their website will of course look great- it is their best work! But seeing a full wedding album will give you an idea of what you can expect from them. Ask them if they have photographed at your venue or similar venues. Do they use flash? Can they shoot in low light situations if you are getting married in a church? Do you have a huge bridal party? What is their experience with shooting large groups? Do they provide a second shooter? Does their package include digitals or do they sell them separately? Do they have insurance? If you have any concerns whatsoever bring them up. A professional will have no problems answering your questions.
4. Don't let price be a deal breaker.
When you are first looking at a photographer's website, the price page should not be the first page you jump to. Good photography is worth investing in. If you are in love with a certain photographer's work, then you can get a more affordable dress, or have grandma cook, or make paper flowers. Most of the stuff you spend money on will be gone after the wedding day, but photos last a lifetime. You'll want your kids and grandkids to see your wedding photos but not if they are terrible photos taken by a family member or someone off Craig's list. On the flip side you could hire a photographer that charges more and think "Oh since they charge this much they must be good," when in reality they may be a new photographer that just pulled a price out of thin air. Pick your photographer based on their work not their price- whether you think it is too much or too little.
5. Finally, go with your gut.
I pretty much use this philosophy for everything in life. If you get a weird feeling about someone even if their work looks great, don't hire them. And if after meeting your photographer, you feel as though you totally connected take it as a good omen!