I had the same question, glad I searched before posting.
Too many appraisers in my area arbitrarily describe it as 100% residential - my market is mostly suburban and rural mind you. Some counties in this region supply studies that have GIS created maps breaking it all down, some do not.
I would suggest going to your county planning commission, or, as TEL2002 said, use Google Earth or MS Virtual Earth as an aid.
Just know your market. Describe the uses of the land within the market area that represents the competition for the subject. If it's out in the country it might look something like 40%-50% residential, no measurable 2-4's or multi-families, a small amount of commercial and the rest Ag or other uses (public lands, recreation, etc.). In a densely developed metro area it might be more like 70% residential, 5 and 5 for 2-4's and multi's and 10% commercial (neighborhoods need a small amount of commercial for convenience services...mini-marts, fuel, medical clinics, etc). and the rest, if any, might be public uses such as parks, greenbelts, libraries, schools, etc.
This isn't an exact science and no one is going to find fault with reasonable estimates.