This time of year is perfect for picnics and other outdoor events, and few items make for better outdoor eating than melons. The melon industry has changed a bunch over the years. When I was a kid, watermelons were 25-pound monsters. Most watermelons sold today are seedless and much smaller varieties, bred to be "refrigerator-sized" and easier to harvest and transport. Recent changes in watermelon production and post-harvest handling have been positive — particularly in the last few years as a re-emphasis on flavor has brought back some great heirloom varieties or has greatly influenced the selection of new ones.
Melons are a member of the Cucurbitaceous (or gourd) family — this is a very large and diverse family of plants that includes cucumbers, squash (winter and summer), and pumpkins. The most common melon varieties produced in the U.S. are musk melons (including cantaloupes), inodorous (or dew) melons, and watermelons.
It can be difficult to determine the inside condition of a melon from the outside, and out of all the melons, it is most difficult for watermelons because of the wide range of varieties. The "thumping method" is certainly the most entertaining (and widely used), but is not the most reliable in my experience. It goes like this: "If the melon sounds like your head, it is too green; if it sounds like your stomach, it's too ripe; if it sounds like your chest, it is just right." This system may or may not work for you.
Most watermelons will have two colors: a dark green with a lighter green or white secondary color. Most will also have a white spot where the melon was resting on the ground when it was growing. This is key because the rind of most watermelons will turn slightly yellow as the fruit ripens. So I look for that slightly yellow tinge and I am rarely disappointed. Of course the most reliable method is to ask your local produce team member to "plug" a melon for you (cut out a small triangle to see how it looks and tastes).
— James Parker, global associate perishables coordinator for Whole Foods Market