Look for a coconut that shows no sign of cracks and is free of mold. Pick it up and shake it. Speak to it in a low, reassuring voice. If it does not respond with any cracks, it has obviously gone flat. Try for one with a quicker wit. It should be heavy with water, which, in turn, ensures a moist interior - always a good thing!
Always buy an extra coconut or two in case one turns out to be rancid, or you decide you like its cute little Mr. Bill face and can't bring yourself to swing the cleaver. Then they can form a rough hairy gang, or everyone can go bowling.

How to break a coconut