An exception is apples, which should be hard. Try smelling fruit. If it doesn't smell all that flavorful or ripe, it probably isn't. Select local produce if possible. Because it hasn't been shipped from somewhere far away, it has a better nutritional quality.
Vegetables are usually available at stores fully ripe.
Fruits, on the other hand, are often sold a little underripe. Placement in a sunny windowsill for a day or two can speed up the ripening process. An exception is apricots, which have a short season and are very perishable. Remember that berries do NOT ripen after harvest. Select only peak berries for juicing and juice as soon as possible.