You certainly can test for some non-organic foods at retail.
For example, non-organic oats are often sprayed with roundup at harvest time, so cereals made from them routinely have unsafe levels of carcinogens in them:
Note the use of glyphosate as a dessicant is highly regional -- it's common in the Dakotas, Saskatchewan and UK because they have growing seasons that are too short for the crops being grown, but nearly unheard of in places like Iowa and Indiana, because the growing season is long enough for grains to mature and dry before the weather turns.
For example, non-organic oats are often sprayed with roundup at harvest time, so cereals made from them routinely have unsafe levels of carcinogens in them:
https://www.ewg.org/childrenshealth/monsanto-weedkiller-stil...
Organic oats have less than 1/10th as much roundup in them on average:
https://gimmethegoodstuff.org/pesticides-in-organic-oatmeal-...
Also, the more farms go organic, the less contamination there will be in the organic crops.