Sites like Zillow, Eppraisal and others will give you a very broad idea, +/- 10% to 20% in some cases, read the fine print on each site for their accuracy in a given area. Contacting a couple of agents or an appraiser can work to give you a better idea. The only true way to find the most accurate and true value of a home is to put it on the market and see what a willing buyer will offer.
Agent's may over promise on a value in hope of getting the listing. They may under promise hoping for a quick sale with minimal work. Appraisers use past data and make adjustments based on how similar your home is to the recent comparables. The more similar your home is, the more accurate the appraisal can be.
Depending on the reason you need to know, any of these can work. If you are planning to sell, don't fall for the promises of the agent who over prices. They often hope you will drop the price over time or the market catches up to their suggested price. I your market is flat or in decline, you will only get farther away from the real market value over time.