Assessing unmet demand in a rental market

[Brian Larson]

What are some good ways to assess the unmet demand of a rental market?  Maybe it's a small town and there are very few homes available to rent and there are several families that would rent there if there were available homes in their price range.  Is there a way to find out how long a rental sat onoccupied?  Is there a way to create an interest list where you could get people to sign up if they are interested in renting in a particular area?  I'm just investigating rental property ownership and this is one of the questions that came up for me.