Is my watermelon ripe?

Figuring out when a watermelon is ripe (i.e., ready to pick) isn't rocket science. See the curlique at the base? In this picture it's green (unripe). When it dries up, the watermelon is ripe.

If the curlique is dry or mostly dry, thump the watermelon. If it sounds hollow, it's ripe. Thump a couple in the area to see if you can tell the difference.

Look at the bottom. The color should be (note the qualifier "should") gold or yellow. This one isn't as absolute as the others, as I've eaten fully ripe watermelons that were still white on the bottom. Different breeds have different habits.

The problem comes in when you're picking one from the grocery store. The curlique got left behind when it was picked, it was probably picked at the same time as all the other melons without concern for peak ripeness, it's probably been bred for storage life rather than taste, etc. At that point, the thumping is the best option, but it takes practice to tell a ripe watermelon from unripe.