You should read the label and see if it is certified by a respected organic organisation like the USDA and the Soil Association. The certification process is very expensive, that's why you pay so much for certified organic produce.
I really don't trust anything label USDA organic. I look for independent certification agencies. I live in California and we have stricter organic guidelines than the USDA.
In several countries, including the United States, there are government agencies that require them to organic food producers to meet certain standards of crop production (not to use pesticides, antibiotics, additives and grown in soil free of chemicals) , storage (separated organic food non-organic), some processing, packaging and transport of organic foods. In the United States, you check the label says "USDA Organic" which is the seal of approval from the Department of Agriculture. If you live in another country, consult with the agricultural control agency. The decision is in your hands. Information is your weapon to make the best decisions about your health and diet.
I agree, that is a really good question. I tend to buy from local farmers as much as I can and I guess I am just taking their word for it. The few times I have purchased organic products from a store, they have been from a local store that only carries organic products so again, I am taking their word for it. I have avoided buying organic from the major store chains simply because they charge way too much.
I have often wondered if all that money I spend on organic food is really worth it... Now, I grow simple stuff like spinach, chard, herbs and lemons at home in containers. At least now I know that I eat real organic stuff. I do not have the space to grow fruits .. I would be happy to grow most of the food that I consume raw.
You don't unless you know where the food came from and unless you know that the place you are purchasing from is diligent about that sort of thing. However, there is a stringent process growers have to go through to be certified as organic and many organic growers don't bother with it. Also, it is important to realize there is a distinction between organic and natural.