From the Old Farmer's Almanac:
Nine Ways to Tell if Your Watermelon Is Ripe
NORTH, SOUTH, East, or West, wherever you grow your watermelon, this summer you'll face the daunting task of determining its ripeness. We've heard all sorts of advice, including the fact that a ripe melon, when thumped, will feel more like a human head than a human chest. Dr. Bill Rhodes, professor of horticulture at Clemson University, offers the following but warns that for the past 20 years, he's been hoping for better insight.
1. Thump it. If the watermelon sounds hollow, it's ripe. This is difficult for less-gifted ears.
2. Look at the color on the top. The watermelon is ripe when there is little contrast between the stripes.
3. Look at the color on the bottom. A green watermelon will have a white bottom; a ripe melon will have a cream- or yellow-colored bottom.
4. Press on it. If the watermelon sounds like it gives a little, it's ripe. (Rhodes doesn't like this method because it can ruin the quality of the fruit.)
5. Check the tendril. If it's half-dead, it could mean that the watermelon is nearly ripe or ripe. If the tendril is fully dead, it could mean that anthracnose or some other fungus killed the melon, or that it's ripe or overripe. If the tendril is green, you should wait to pick the melon.
6. Count the number of days from anthesis (flowering) or the number of days from planting. This works pretty well if you know the variety of watermelon and how many days it's supposed to take for that variety to ripen under normal temperature and fertilizer regimes.
7. Check the size. It's not necessarily true that when a watermelon is big enough, it's ready; but under good conditions, it should be normal size. If it's not, you're probably too anxious.
8. Crack a few. You've got a whole field of watermelons, and you can practice a little, right?
9. Is the vine dead or dying? Well, the watermelon is not going to get any riper, so you might as well pick it.