How does one determine the demand for condo rentals in a certain market?

Can you answer Curt F’s question about Timeshares?:

I am considering buying a couple foreclosure condos and renting them out, however I am finding it difficult to guage the rental market in this particular area. Does anyone know of any sources that might help me see what I could probably charge for rent and how many people will be interested in the place?

Fractional Ownership Timeshare