Melons can be a little tricky. At the farmers markets, you can rely on a Red Hat Melon vendor to tell you when a melon is ripe or if you can eat it today, tomorrow, or in three or four days. But here are a few tips for when you are on your own:
Cradling the watermelon with one arm, smack gently with the flat of your hand. A ripe watermelon will have a vibrant resonance. If the melon is overripe the thump will feel dull and flat; if it's underripe it will also feel flat, but with a lighter tone. Check the ground spot: in general it will be yellow when ripe. If you are picking a watermelon in the field, look for the first leaf and first tendril to be dead or dry.
In general, we tell by color: a cantaloupe will usually go from green to gold through its stages to ripeness. Look for gold or beige under the netting; smooth-skinned cantaloupes may do a different color change depending on the variety. Some need to be cut from the vine to avoid pulling out a piece of flesh, but some "slip": in this case, check the stem end to see that there is an indentation where the melon released from the vine. In the field, this type of cantaloupe will release easily from the vine; leaves near the crown will begin to brown and the underlying color of the melon will begin to lighten before the melon releases from the vine with a gentle tug. A ripe cantaloupe generally has a pungent sweet scent, especially at the stem end.
You should never press into the stem end or squeeze a melon; this will hasten its deterioration and is unnecessary as an indicator of ripeness.
Mike and Rose Marie Nichols of Nichols Garden Nursery talk about how to know when a melon is ready to harvest: