Watch for the icebox watermelon rind to change from a bright green color to a dull green color.
Look at the stem where the icebox watermelon is attached to the vine. The curly tendrils that grow from the stem turn brown and become dry when the melon ripens, according to University of Illinois.
Lift up the watermelon. The area where it sits on the ground will be a creamy yellow color when the melon is ripe.