From asking around, canvassing the neighborhood for "for sale" signs, and checking Web sites, you should have a list of a few promising agents. Before hiring one, you should conduct an in-depth interview with three or four. Call ahead and make appointments, letting each agent know that you would like to see a comparative market analysis (CMA) for your house, and a list of their most recent sales (sometimes called an "activity list").
Do some research into the real estate market in your city before you meet with the agents. Web sites like Zillow.com and eppraisal.com are useful resources for guesstimating your home's value. At the meeting, the agent should present their CMA, a suggested market value for your house based on recent comparable sales. Comparable sales (or comparables) are homes that have sold recently and are similar to yours in size, location, age and amenities. The agents you speak to should come up with CMAs that are in the same ballpark.
Remember: You're not looking for the agent who presents you with the highest selling price. Often, agents will attempt to "buy a listing," or promise an unrealistic price high above market value to lure you into giving them your commission. You want to see a CMA that seems reasonable based on your own research, and that includes homes that are actually similar to your own. You don't want an agent who presents homes extremely different in size and location from your own in a CMA. They either don't know the market, or worse, could be trying to manipulate the CMA to get a higher value.
The agents should also show you activity lists that represent their recent completed sales. You want to look for quantity, to make sure they can move multiple listings at a time. You should also look for quality. Good signs include listings that sell quickly, and final sale prices that meet or exceed the listing price. Also, ask the agent about their marketing plan for your house. They should be willing to put ads online and in local papers, offer ideas for showing your house and not be afraid to spend money on marketing to make a sale.
Finally, personality is important. Remember that you'll be working very closely with this person on one of the most important decisions of your life. You shouldn't pick someone you can't stand to be around.