Fruit – How to pick a watermelon at the supermarket

Depends on if you'll buy for instant consumption or for storage.

If you want to eat/prepare them right away, you want an avocado that feels soft on the inside, when pressing them they will cede and have that soft feeling ripe avocado has. It shouldn't cede very much though, as those are past their prime. The very good ones even smell in a nice avocadoey way a bit (or maybe that's just my insanity.)

If you want to prepare them later, you want firm ones that cede only a tiny little bit (the rock hard ones mostly never ripen in my experience,) you then store them in a porous bag (the typical brown paper bag or a newspaper) to let them ripe. To speed up the ripening, you can add an apple to the bag.

See also this related question

In the case of most bush berries (blueberries, raspberries, blackberries) depth of color and ease of picking are the most indicative signs of ripeness. The berries that are on the sunniest side of the bushes will ripen first. The greater more sun exposure they receive the quicker they ripen, thus the reason that a grouping of berries even in one area will ripen at different rates.

Many people say that the larger berries are sweeter but my experience is that is more anecdotal than preciese. Most fruits will be more plump with access to more water during the growing season but that doesn't necessarily make them sweeter. Grapes that have to struggle generally tend to produce better wines with more concentrated flavor because the natural sugars aren't watered down by high quantities of water.

To reiterate, look for deep dark color and ease of picking.