Steve posts some useful links below. It is my experience the very best way to get to the bottom of the current market rents is to take the time to drive through the surrounding neighborhoods that are in the same school areas and simply call on the available rentals. This gives you not only the rental rate it also allows you to talk with other rental owners and get the vacancy rate, deposit and other customary move in fees. Doing this has been helpful in my client's decision making process when buying investment property.
It is also very important, both in buying and selling, to make calculations using both cap rate and gross rent multiplier equations. A well designed Excel spreadsheet will make this VERY easy. I have found using some net factors to be best for planning investment strategies. Using additional data including repairs, maintenance and loan costs into my spreadsheet has proved to be an invaluable way to get an accurate analysis.