Delicious Living magazine provides some tips on buying organic produce. I've added a few remarks of my own to this list.
(1) PLU (Price Look Up)
The PLU sticker features a code number. If it starts with a 9, followed by 4 other numbers, it's organic produce. If it starts with an 8, plus 4 other numbers, it's genetically modified (GMO) produce. If the PLU has only 4 digits, it's conventional, non-organic, non-GMO grown fruit.
Select larger fruit early in the season, then mid-sized fruit in late season.
Fruit is fully ripe when the bottom (calyx) of the fruit is fully colored, rather than dim or pale. In general, a bright, evenly colored sheen is best.
Exterior of fruit should be firm, not soft, and free from blotches or blemishes. However, some freckles or "blush" is okay. Shun fruit with withered, sunken, or squishy spots. Uneven or unusual colorings are generally not a problem. Potatoes should not be wrinkled or sprouting. Chile peppers should not be blemished.
Hard fruit, like apples and pears, should be stored in the crisper, away from other strong smelling foods, like onions. Fruits and vegetables will ripen more when left at room temperature. Keep an eye on grapes and berries, which can tend to mold, especially when wet.
Parents, get your kids involved. Take them with you when you're grocery shopping and make it fun. Challenge them to find different colors of fruit and vegetables. For example, say "Okay, now let's find something orange to put in a salad and something blue for a fruit cup." Teach them why fruits and vegetables are better than candy and junk food.
More specifics at "Shopping for Fresh Vegetables: A Guide"