. . . in case they come in handy for anyone reading this.
1. Unlike many other fruits, strawberries do not ripen further after picking. If you buy green bananas they will last longer. If you buy green strawberries they will still be green when you either eat them or throw them away. Generally darker berries will be sweeter. You can tell that strawberries are going bad if there's mold (of course) or if they're starting to get drippy.
2. When buying corn on the cob, a dry husk does not necessarily mean the corn is bad. Look for plump kernels without dimples. The cob should be filled out all the way to the end. A fat, heavy ear of corn is better than a slender, lightweight ear.
The sugar in sweet corn begins to turn to starch the instant it's picked, so the less time between picking and cooking the better. You can't always tell fresher corn by looking. The bright, pretty green ears of corn just out of the produce cooler might be every bit as old as the ones that have been sitting out on the sales floor for three days while the husk turned papery. Your best bet is to find out where the corn is from. Corn that was grown close to wherever you are will always be better than corn that has been shipped in from far away.
Finally, if you are putting ears of corn into a plastic bag, especially one of the thin ones at the supermarket, put the corn tassel side down and it will not slice a hole in the bag.
3. When buying watermelons, the rind should be firm and not have any give to it. Yellow on the melon means it remained in the field long enough for the sugars to start coming through the rind. The more yellow, the sweeter the melon will be. Don't thump it for sound while it's laying down, or on a pile of melons. It will pick up the tones from its surroundings. Pick it up and give it a solid rap with the tips of your fingers. You will not be able to tell anything by slapping it gently, as I see so many people do. What you're listening for is a good, hollow tone like a drum. I like for watermelons to sound with a medium C or B flat tone, maybe a D. It should echo. The more high-pitched the tone, the greener the melon. The deeper the tone, the riper the melon. If it just gives a dull "thud" then the melon is overripe and the flesh will be mushy. Usually, by this time, the rind is starting to go soft as well.
4. If you're buying fruit that is pre-packed in a plastic bag and if it's not cold, avoid buying fruit that has moisture droplets inside the bag. If the bag has just been brought into a warm room from a cooler, the moisture is probably just condensation. But if the bag is room temperature and there is moisture, it means something in the bag is going bad.
5. Granny Smith and Jonathon are classic baking apples; however, you can cook with almost any variety of apple except Red Delicious. It has a high water content and will turn to mush when heated.
6. If you need potassium and don't like/can't get bananas, never fear. A serving of kiwifruit (two fruits) has more potassium than a medium banana and also more vitamin C than a grapefruit.
7. When buying grapes, the rule of thumb is that darker is sweeter. Red is sweeter than white/green and black is sweeter than red.