The prime season for raspberries is from midsummer to early fall. Plump raspberries that have a rich, deep color and a sweet, berrylike aroma are the most likely to be sweet and succulent. Avoid berries that have a hull attached in the center, which is a sign that they were picked before fully ripening (which makes them more tart), as well as those that are prepackaged too tightly and/or have signs of stains and moisture, for these berries are more likely to be crushed or moldy.
Raspberries are extremely perishable. Plan on buying small quantities and using them within a day or two. Never wash berries before storing or keep them exposed to sunlight or at room temperature for too long. Remove any crushed berries, because these will mold quickly.