How should I determine the fair market rental value of our (furnished) home?

My wife received a parsonage allowance for the first time in 2015, and I understand I need to document the Fair Market Rental Value of our home (furnished). What are the most common acceptable ways of doing this?

I know one way would be simply to ask a real estate agent to compute it, but roughly how much would they charge? Is that what most clergy in our position do? Just starting out...

Jim