Assuming that the durian is not frozen, the first thing that you should do is pick the durian up in both your palms. Do this gently, and if the thorns hurt, use a couple of tea towels to protect your hands. Place the durian next to your ear, and give it a gentle shake, not unlike shaking a maracas to some soul music. If there is no sound, it means that the fruit is too ripe. If there is a hard knocking sound, the fruit is not ripe enough. What you want to hear are dull, subtle knocks.