You'd be entitled to get a few soil and water samples for analysis. You could then take those samples to your county/city office and seek further advice. Let them do the run around for you or at very least, advise you on the next step - maybe there'd be some costs involved but it might be worth it. I'm not a US citizen, but this is what I would do in my home country if looking to buy land in a crop-growing area.
I don't know the restrictions on ag-chemicals here but I would assume the toxins used on the crops you mentioned would have a fairly short half life. There will always be residue from these chemicals - even if they are supposedly 'biodegradable'. So I would advise caution, but not paranoia. You could well be onto a good thing for a good price. Just be sure to do the homework.