I don't know that there's any one source for figuring out your target demo. It really depends on so much - location of course being the main factor. You really have to know what's happening in the areas where you're buying. Some areas are on the decline. Others stable and others up and coming.
What other development is taking place around you? What's the larger plan for the community? Is the area becoming popular among the young and adventurous? Is the area predominantly Hispanic?
Marketing a property in North Park is going to be a completely different experience from marketing one in National City. North Park is going to be more of an upscale experience to the yuppie/hipster crowd while National City is going to be more of a budget minded crowd. It certainly helps to be involved with the community where you're working so you know what's going on. In the past, I sat on the Business Improvement District board where my studio is located. I also sat on the redevelopment committee so I knew who was doing what and when - and I knew it before most other folks.
For example, knowing that hip restaurant was going in at a certain corner means that a similarly hip clientele is likely to be attracted to that area and more growth would take place. I don't know that you can just sit back and think that looking at some reports or numbers is going to give you the ears-to-the-ground kind of information that gives you a competitive advantage over everyone else in the game.