If the sound, by snapping with your fingertip (or knocking with your knuckle), is sharp and high, the melon is immature. If there is a dull, hollow sound, it is more likely to be ripe. When the tendril or curlicue by the melon is alive and green, it is almost certainly immature. If the curlicue is dead, the melon is at least approaching maturity. Another test is to watch the change in color, a yellowing of the spot where the melon rests on the ground. A ripe melon, when pressed with a bit of weight, will usually crackle, but this is not good for ones that are to be kept. There is a variety called 'Golden Midget' that turns yellow when ripe.