I think that is part of why it is so important to buy locally. If you develop a relationship with growers in the area, you have a better chance of knowing what is going on in their outfit. I know that, around here, the farmers who are using the most sustainable methods are well-known for that in the community.
If it's packaged food and it states organic, there are certain conditions the manufacturer of the food must meet to put that wording on the packaging. At the farmer's market you are really at the mercy of what the grower tells you though, so it is good to get a recommendation if you're unsure.
It is also possible to be inspected and monitored, which gains you certification. This should provide peace of mind for concerned customers. Of course the term 'organic' is quite wide, so it's worth doing some research and asking a few questions if you have any particular issues in mind.
When I'm buying packaged food at the store, I look for the USDA organic label. Local farmers have usually been pretty honest when they don't grow food organically, and I can always tour the farms and ask lots of questions.