Most often at the coke plants it was benzene (and other volatile organics) and heavy metals (lead, arsenic, mercury). The former forges and steel plants were metals with some toxic solvents (they liked to have the metal look shiny).
Yes, if you have healthy growth, then most likely you don't have a problem at the surface. (note: at the surface) I would recommend heavy mulching/amending/adding and building soil, and don't disturb the subsurface unless you have to. Nature will tell you whats going on. Pay attention to plants that have deep tap roots, if they don't survive in a certain area, note that, you may have a problem 2-3-5+ feet down.
Any I would agree, fungi, bacteria and yeasts will breakdown everything, it is just time. (its called Natural Attenuation in the environmental cleanup world)
As for the lab stuff,
http://www.epa.gov/osw/hazard/testmethods/sw846/pdfs/8260b.pdf (volatilizes in air (smelly), bunch of stuff from acetone to MEK, to Nitrobenzene, to Vinyl Chloride.) ($85)
http://www.epa.gov/osw/hazard/testmethods/sw846/pdfs/8270d.pdf (non volatile (cant smell it), like PCBs, most of the rest of the bad chemicals that are not in 8260 and not metals) ($125)
EPA 8015M will mainly be used to tell you how much heavy Petroluem is in the soil (light petroleum is in 8260) ($45)
Cam 17 is 17 metals that the state of California uses to determine what is hazardous. Antimony, arsenic, barium, beryllium, cadmium, chrom (total), cobalt, copper, lead, mercury, molybdenum, nickel, selenium, silver, thallium, vanadium, zinc. ($8-10 a metal) or $170 for all
