There's a reason your mother never told you the single, all-purpose way to know when fruit is ready: There isn't one. "There are more than 50 different species, and they all mature in different ways," says David Karp, a Los Angeles food writer. Bananas and avocados are picked green and ripen off the tree, for example, while oranges won't ripen post-harvest. But there are certain tricks you can use. "Ask a knowledgeable produce manager what's in season," Karp says. "Most peaches taste best from late June to early August, for instance." At farm stands, aroma is an indicator. "Fruit should smell as good as it will taste," says T. M. Gorman, the author of Fruit — The Ripe Pick (Vine Publishing, $13, www.amazon.com). But scent won't work in supermarkets, since refrigeration stunts the bouquet. Color is also a tip-off with certain fruits. Ripe peaches have a rich, yellowish undertone, as do cantaloupes, while watermelons should have a whitish yellow underbelly. They should also resonate when slapped, Gorman says, "like a musical instrument."