How do you determine rental demand?

[Richard Arakelian]

How do I go about finding the rate of rental demand in a certain town or county? 

I'll look at some areas and I won't see many houses for rent, while in others I see many. 

How do I know if those areas with few rentals are places that no one rents or rentals are needed, versus those areas with many rentals which have too much supply or everyone rents there? 

Is there a metric that shows rental demand rates in given area?