Formula that indicates a good rental market.

[John Mitchell]

My wife and I have been investing since 2004.  We buy and rent out homes.  We currently own 11 homes and 1 4-Plex.  All are properties are in one city except 1.  My question is how do you guys determine if a market is good for rentals?  Meaning, how do you know there is a demand in a community?  We just got real lucky with the area we chose by accident.  We want to buy a 41 unit, 68 unit, and 12-plex in another city.  It only has a population of about 11,000 people, which is smaller than we are use to.  The 41 units is 85% full, 12 unit 58% full, and the 68 unit is 82% full.  We feel it is not well managed and that with our management style, we could fill the units up.  However, what if there is a decline in the demand for apartments?  How will I know?