When determining a fair market rent for a property, one useful resource is the U.S. Department of Housing and Urban Development (HUD). The agency typically has data regarding rents in any given market throughout the country.
This does not give an exact value, particularly since appropriate rents will vary depending on the neighborhood and condition of a property, but can be a useful starting point.
Last, consider hiring a property manager to help you calculate fair market rent. Among their other services, property management companies should be able to calculate pricing for housing that they are operating. When you're ready to start your search, All Property Management will be here to help.