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Quite right that the older and newer, but cheaper pesticides have a longer half-life than expensive, modern ones. I'm unsure as to how chickens metabolise pesticides (but DDT was shown to weaken eggs shells, i recall). I know that in humans, residual pesticide compounds tend to accumulate in fatty tissues, but thats about all.
The only way to be sure would be to have some soil samples taken and analysed, but that won't be cheap! I guess you could read up on the residual characteristics (half-life) of common pesticides used in your area a decade ago (farm supply store owners would know) and take it from there - but even then, without knowing the kind of dosages used, it will still be an educated guess.