Inspect the cranberries visually, looking at the berry coloring. Ripe cranberries are bright red or scarlet.
Look for berries with a firm, taut skin -- more indications of the fruits' ripeness.
Place the berries in a bowl full of water. The ripe berries float to the top, while those that are not mature remain at the bottom. The fact that ripe cranberries float is one reason they are grown in bogs or in fields that are later flooded, allowing the ripe berries to float to the top.
Bounce the berries on a hard surface such as a kitchen countertop as a final test if you still cannot tell whether a cranberry is ripe. A ripe cranberry bounces, while those that are not ripe make dull, thudding sounds.