I have to agree with what another poster said about farmers markets. There's vegetable and fruit stands that are good and some that are bad.

To anyone that has ever grown there own tomatoes fruits/vegetables, then you can taste the difference between small fruit/vegetable farmers who use pesticides and modifiers verses the ones who just use cow shit,water,dirt,and a little friendly help from the Sun. Another idea is you can start your own garden. I would hit the fresh fruit/vegetable stands first along with buying seeds to grow your own stuff. And after just tasting the produce you can tell whats modified or not. I know I can.
But after a year of your own growing you can then finally rest assure your produce is clean.