One cannot readily take a refractometer into a grocery store to measure the Brix of fruits and vegetables in the produce section, and perhaps not even the local farmer’s market will allow produce to be cut and tested before purchase.
So how to tell what’s best when you can't (or don't) grow your own? I don’t know that there IS a good way, but I came across this information in my files. Unfortunately I had no source noted, but it probably came from Rex Harrill who posts excellent information for growing nutrient-dense foods. I have not tested the reliability of these suggestions, although they seem sensible to me. I DO know my nose isn’t sufficient for choosing whole melons by smell!
VEGETABLES: A natural waxy coating is good. Packers, processors, and stores try to duplicate this effect by mechanically waxing poor quality vegetables. Any hollowness indicates a mineral deficiency (probably boron).
POTATOES: Sunken eyes signify lower quality (probably short on manganese).
PEARS: A boxy shape is better.
STONE FRUITS: A split pit indicates poor quality and mineral insufficiency.
CITRUS: A thinner rind indicates higher quality.
ANY ITEM: Bright pure color, whether in cut flowers or cut watermelons suggests higher quality. Slime or mold can be washed off the surface, but it will have grown throughout the item. Reject such food.