Examine the spot where the melon has been resting on the ground. Scratch the surface of the rind with your thumbnail. If the outer layer slips back with little resistance showing the green-white under the rind, the watermelon is ripe. Scratching unripe melons only leaves a darker depressed line. Choose a melon with a smooth surface, dull sheen and well rounded ends.
Many people purchase cut melons to judge ripeness from inside appearance.
The more red flesh and less white rind, the riper the melon. White seeds usually indicate the melon was picked too early. Storage Watermelons, if not too mature, can be ripened for a few days at room temperature. Wrap melons in waxed paper or in secured plastic bags before storing in the refrigerator, as melon aroma readily mingles with other foods. Watermelons can be stored uncut for 2-3 weeks.