How to determine if there's demand for your prospective rental?

[Steve S.]

I'm looking at acquiring my first rental property.  Is there a typical way one goes about determining if there's demand for your rental?

The numbers on this single family property look to be pretty good if I can just get a good tenant in it.

How does one go about determining demand?  Should I post a craigslist ad?  Is that illegal if I don't own the property?

I could talk to a realtor, but I'm also looking for something more empirical than a realtor's opinion.

Any suggestions?