The underlying soil varies throughout the City and surrounding areas. As a general guideline, the further west you get, you are more likely to be on solid bedrock, whereas the further east you get there is a greater likelihood for significant clay in the soil. The expansion and contraction of the clay is one of the major contributors to shifting foundations. As another contributor pointed out, ensuring consistent moisture levels around the perimeter of the slab can go a long way towards preventing any major shifts.
You really have to look at it from a neighborhood by neighborhood perspective. A knowledgeable agent can help you on that front. There are also several large, reputable foundation repair companies that provide free foundation inspections. Be aware, though, when your company is in the business of selling foundation repairs, it's difficult to find a foundation that does not need repair! Unless you are sure the slab needs repairs by visual inspection, and thus need the bids for repair, I would consider using an independent structural engineer that does not repair foundations to inspect the slab. It will cost some money, but you can be sure that you are getting an unbiased opinion.