I know many ways to measure local supply, but not local demand.

I think one way to measure demand is to observe jobs openings. This can be found on job-post websites or through local newspapers. I can even look through past newspapers to study past demands.

Another way I think I can measure demand is through some theory of needs and existence. The theory is that something (or someone) doesn't exist if its needs aren't met. For example, I live in a neighborhood of houses. A house needs certain maintanence or else it will no longer be a house. So that certain maintenance becomes a need with an assumed demand.

I can also observe demand through the law and governments. Sometimes the governments have requirements for certain things. For instance, sometimes citizens are required to file taxes. Some immigrants are required to pass examinations to become citizens. Spare drivers are assumed demanded to help drive intoxicated drivers who legally can't drive. Governments and law can have an indirect effect on demand.