None of the farmers use a GAP program (on farm food safety)?
You should ask if they follow
Harvest date from application standards
Application rates of chemicals follow the particular manufacturers guidelines
Is sewage or compost applied and if so how and how far ahead of harvesting is it used
Are they using a GMP seed or not
Do they follow good harvesting practices if harvested by hand
Have a look at the Global Gap standard or the USDA on farm food safety program and then base your supplier approvals from there
Even if the farmers aren't certified, they are probably doing everything included in a GAP program