I have a small back yard on what used to be a historically heavily industrialized area. So I expect the soil to be full of PCBs and what-not. On the other hand, I know the previous owners had luck growing (and failed to die from eating) their own vegetables here. I'd like to plant a small garden, but am worried about bad (ie polluted) soil. How should I go about testing the various parts of the yard to find out where they may have brought in new soil, and where there's 100s of years of engine oil lurking?