Cold damages tomatoes and stops ripening, so as a rule, never buy tomatoes from a refrigerated case.
Tomatoes displayed loose are easier to evaluate than those packed in boxes. Look for plump, heavy tomatoes with smooth skins. They should be free of bruises, blemishes, or deep cracks, although fine cracks at the stem ends of ripe tomatoes do not affect flavor. Make sure the leaves of greenhouse tomatoes are fresh and green.
Ripe tomatoes are fragrant, but even mature green ones should have a mild fragrance that promises future ripeness. If they have no aroma at all, the tomatoes were probably picked when immature, and will never ripen.
Fully ripe tomatoes are soft and yield to the touch; buy them only if you plan to use them immediately. Overripe tomatoes, provided they are not moldy or rotting, are perfect for making sauce, and even briefly cooking fresh tomatoes releases their lycopene. Choose whatever size tomatoes are appropriate for your intended use. Size has no bearing on flavor, texture, or quality.
A note on buying tomatoes in winter: Year-round demand for tomatoes has led growers and breeders to develop thicker-skinned hardy varieties that can withstand the rigors of machine harvesting and long shipping distances. Sadly, commercial breeders did not include flavor or sweetness in the list of desirable traits to breed for. Because of this, and because of the way these tomatoes are handled on their way to the market, their taste, texture, and juiciness invariably are a poor imitation of tomatoes available at farmers’ markets in peak season. Still, it’s possible to get a winter tomato with a decent flavor in the off-season, especially if it was grown and shipped under the right conditions.