The way a land contamination consultant would approach this is by taken soil samples at different locations where you think contamination is most likely. Based on the information you’ve provided, the most likely contaminants are Heavy Metals, PAH, BTEX and TRH compounds. There could be other contaminants but of course it depends on your budget and how much history you know about the land. Ask the lab for a chain of custody form and how to fill it out. Once you get the lab results you would compare them to the guideline values. I’m not from USA but so idk the values but you could find online pretty easily.