There are several ways to find out how much a property is worth.
The "in-depth" way to do it is to do an analisys of comparable sales. We have a special, proprietary spreadsheet model that allows us to evaluate the last 100+ sales in an area based on the price per square foot (PSF), tax assessment ratio, and other factors. Here's a screenshot:
A quick way is to look for recent sales in the neighborhood. The Washington Post has a great site for this, available here.
Many of our customers ask how good sites like Zillow.com are. Our answer is, "they're good, some of the time." Our experience (and backed up by a recent WSJ article) is that the estimates can be very accurate, or off by more than 25%, and it's hard to tell when they're wrong. We think Zillow is great to get the feel of a neighborhood, but very dangerous to use when deciding how much to offer or sell for on any one specific property.