What you require is called a Comparative Market Analysis within the industry. It is comprised of data based on properties within a geographic radius of your property. A good agent will "pull" properties that have recently sold (within 6mths), properties that have just gone under agreement and properties that are active, withdrawn, cancelled, etc. With this information you and your chosen Realtor should be able to make an educated decision about your property value.
At that point you might want to interview several agents. Each will likely have a slightly different opinion in terms of value, and each will have a different approach to marketing. Choose the agent you are most comfortable with, has the best sales statistics and marketing strategy. Be very careful not to get fooled by the wrong numbers. It's not necessarily important how many homes a given agent has sold, but rather how quickly, and at what percentage of the original price. Not the most recent sales price. You're looking for accuracy and efficiency.
For instance, for 2008 my team has a 100% (on average) sale price:original price ratio with an average of 29 days on market. You would be hard pressed to find an agent with those statistics in this market. Good luck with your property. Call or email for assistance anytime. We can provide a list of Charlestown references upon request. It is very likely that your unit is worth a good deal more than what you paid for it in 2003. ;)
There is no substitute for a good agent. They will easily pay for themselves. There are however, many substitutes for a bad or mediocre agent. A monkey, for instance...