It is challenging to be 100% sure on anything relating to food because the research is always changing, technology is always changing and so is the way our food is processed and studied! Balance is always key, eat at least 80% of your food raw and from there some of it organic...
The best advice I can give is to buy what you can organic, particularly foods that more readily absorb pesticide and chemicals. My sister inspired me towards this thought process, as she has this list hanging at home to remind her where to place her priorities financially at the grocery store.
The Clean 15
You do not need to buy these organic because they are less likely to have absorbed any harmful chemicals and some of them may not even need chemicals to grow.
The Dirty Dozen
This list is where you want to focus your attention when buying organic and it is possible to find these foods for the same or close to the price as their un-organic counterparts.
CLEAN 15 - Click Here to open a file that you can print & hang on your fridge or place in your wallet
DIRTY DOZEN - Click Here to open a file that you can print & hang on your fridge or place in your wallet