A real estate professional will look carefully at recent homes sales and pick like properties and amenities to help determine value. Alternatively, you could hire a licensed appraiser to review your home. The online programs such as Zillow or Cyberhomes use proprietary data to arrive at estimates, but reviewing the discussion boards on some sights leads to questions as to the relliability of the data.
Your best bet would be to talk with a experienced realtor in your area. Many will give you a no obligation, free market analysis based on recent activity in your immediate area. This information tends to be more accurate than Zillow and other such programs. Please contact me if I can be of further assistance.