I want to start with a disclaimer: I am not an expert ripe watermelon picker, and, just because a watermelon is ripe doesn't mean it will be perfectly sweet. I love growing and eating watermelon, but I have struggled with picking and then selling watermelon for the fear of cutting into an either green or over-ripe watermelon.
Oh, there isn't anything like eating a cold, sweet watermelon on a hot summer day. It's almost better than a glass of iced tea when you've been outside working and in need of refreshment. When you cut open that watermelon, it can be disappointing if it isn't at it's peak. Personally I do not mind a melon slightly under-ripe, but my palate is disturbed by a gritty over-ripe melon. An over-ripe melon also won't keep at all, where-as a perfectly ripe or slightly under-ripe one will keep for up to a week in the fridge if kept in the rind.
At the Market:
If you are purchasing a melon at a market there are several indications of a ripe melon. Always look for a healthy looking melon with a green stem. If the stem isn't green it has been picked for awhile or came off of a dead vine. The melon should be the right size for it's variety, a smaller one could possibly be under-ripe (picked early along with larger more mature melons). If you rub your hand over the melon it should be slightly bumpy, a very smooth melon is probably immature. The underside of the melon should be creamy yellow (not green). Yes, thumping is good. Thump as many melons as you like. If it sounds hollow and not high pitched, it is a good indication of ripeness. In the old days small “plugs” would be pulled out of melons to check for ripeness, but that is rarely done in this day.
At Your Garden:
If picking a melon that is growing in your garden a few more items can be considered. Keeping track of the date of germination and using the maturation date of that variety can help if you grew them from seed. Something that I find very helpful is looking at the curly Q's on the vine. The curly Q growing closest to the melon should be dried up when it is ripe. Thumping is even better at home because the tones of different melons can be compared; the more lower pitched or hollow sounding tone should be the ripest. If it is a large patch of melons, the riper ones will have a matte sheen, while the immature ones will look more shiny. I know farmers that grow large patches can just look out over their melons and know which ones are ripe.
We had another fun technique that someone told us about at the farmers market last summer. Take a piece of straw (a straight, dried stem will work) and lay it on the melon. Give one end of the straw a flick and if it will spin on the melon it should be ripe, if it just toggles back and forth, try another melon. Don't ask me the physiology of this one, but it must have something to do with the water content!
We are still waiting for local watermelons to ripen here in the Ozarks. I just can't wait to cut open my first one of the year! Enjoy your local melons and next time you're at a local market, ask the farmers how they know when a melon is ripe. I'd love to know some other tricks!