Here are some hints on how to buy the best fruit.

Hop

• Oranges/grapefruits:  Squish them a little; if they have some give, it means their rind is thick and they've had time to mature. Or, as with grapefruits, weigh them in your hand; if they're comparatively heavy, they're nice and juicy. (Note: contrary to popular belief, the most orange oranges are not necessarily ripe; it only means they were grown in colder temperatures.
• Cherries:  Look for deeply-colored, firm (but not hard) cherries with green stems (as opposed to dark stems which will be old).
• Watermelon:  Look at the discoloration, where the melon has rested on the ground. If it is big, broad discoloration (whitish-yellow) along the bottom of the melon, it's been sitting in the sun a lot, ripening, and it's perfect. If it's just a small spot of discoloration, leave the fruit for someone else. There is also the "lots of bugs bite scarring theory." It goes like this:  If there are lots of bug bite scars on the melon, it will be sweet. (This theory originated when my brother sold watermelons as a scout many years ago.) These are two good methods. To me all watermelons sound the same when thumped, I threw out the thumping method a long time ago.
• Peaches:  First, smell the peach to make sure it smells good; if you can't smell anything, it's not ripe. Then feel it to make sure it is tender (but don't push too hard).
• Grapes:  The best, crisp grapes will be lighter than those that are squishy and old; pinch a few on the bunch to see if they are firm. For ripeness, look for flecks of tough, brown skin, which are signals that the grapes have had enough time to ripen in the sun.

(Sources: LDSLiving, July/August 2010, and me)