To expand just a little... If the watermelon sounds hollow (if you hear a dull thump/thud), the melon is usually ripe, but this takes some experience... a more reliable method is to look at the side of the melon that rested on the ground. This should be cream or yellow colored, maybe even browning but not white. Additionally, look at the stem where it goes into the melon. If this drying or looks completely dry and especially if it has begun to crack, the melon is ripe. If it dries while the leaves and rest of the vine looks good, this is also a good indicator of ripeness... Good luck!