The most common way that Agents will provide you with an estimate of value, is through the Comparative Market Analysis (CMA). The CMA is a summary of how your home compares to others that are currently on the market, and more importantly, those that have recently sold in your area.
The CMA will take into account the things that make your property unique, and you should see adjustments in value for those things that are either better in your home, or that make yours less desirable when compared to the other similar properties around. The CMA is, of course, still a subjective document, but you are relying on the experience, market knowledge, and opinion of the agent that you have engaged.
There are other ways to evaluate the value of properties including determining the cost to recreate the property, and by examining how much income the property either does, or could derive, however the comparative model is by far the most common.
While the CMA is typically based on what homes have sold for it is not the same as an appraisal. The function of the CMA is to tell you where you should price the home so that it generates enough interest to receive offers. So often, in changing market conditions, you will find that although past sales suggest a certain price, your agent will potentially advise that you use a slightly higher or lower price, based upon the supply and demand of other homes that are for sale in the area.