How to measure if a market if over or under built with rental properties?

[Brett Merrell]

When I evaluate a market for building/purchasing rental property, I have a set of criteria that I use, such as population growth, average income growth, home value growth, future job growth, etc.   However, I do not have a measure to show me if a market is overbuilt or underbuilt with rental properties.  How can I determine this?  Do you know of any data that I can source to evaluate multiple markets?