Great replies to this post!!! I had my own ideas, but always like to compare what I am doing with what the "pros" are doing, especially on researching their sites. For schools, I have had the best luck from those built in the 1920s and 1930s, especially for some silver. The ones that have been remodeled and are still operational schools seem to have a lot more newer trash though. But.....will have some decent clad too. I actually use the 1940s as a cut off for my home sites, since they usually don't get as much traffic as schools, or parks. You need a few more years for them to be "in service" to collect more of the good stuff....just my opinion though. I LOVE the older sites, built in the early 1900s, but tons of iron trash, and harder to find in a big city where they "push em down and build new ones on top of them faster than you can blink"!!!
I have found that most of the counties which contain larger cities, now have GIS mapping systems with overlays for aerial photos, tax records (with property owner names and addresses), tax maps showing property boundaries, and the dates when the buildings were constructed. In fact, I looked up a local school here last night. I had detected a couple of the athletic fields there, but had not found any silver yet. From the architecture of the school, you can tell that some buildings are new and others are quite old. It looked to me like there were 4-5 different building periods there. Well, when I pulled it up on the county tax records, the first 3-story building was built in 1930, the old gym building in 1952, the cafeteria in the 1970s and another remodeling and addition in the late 1980s. So, the key to that location is to figure out which areas were used originally as the athletic fields, etc., and which have not really been disturbed since that time by the new additions, remodeling, etc.
Anyway, keep the posts coming. I would be interested in hearing other opinions on this and any good ideas or tips for research!!!