Smell is perhaps the best indicator when most fruits have reached their peak. A ripe fruit will smell good and have a fairly strong aroma. Here are some specifics:
Pears, peaches, nectarines, plums and mangoes are ripe when they have a pleasant aroma and are just slightly soft.
Bananas are ripe when the skins turn golden yellow and develop brown flecks
Pineapple should remain firm and not mushy. It will have a pleasant pineapple aroma and the leaves on the top will be easy to pull off.
Cantaloupes will be heavy for their size, and the stem end will smell deliciously of cantaloupe.
Watermelon will not ripen any further once it has been picked, but a large yellow spot on the bottom will mean it's ripe.
Apples will be firm with no blemishes.
Berries will be fully colored and plump.
How about vegetables? Most vegetables such as squash or lettuces, can be used at any stage from immature to ripe.