Examine the area of the vine closest to the watermelon. There will be a tendril there known as the "pigtail." A ripe seedless watermelon will have a brown and dried pigtail, with the rest of the vine and leaves look green and healthy.
Turn over the watermelon and examine the area where the melon rested directly on the soil. This area is referred to as the "ground spot." A good ground spot should be yellow or vanilla-yellow, if the watermelon is ripe. A green or white ground spot indicates an unripe watermelon.
Make a fist. Rap on the watermelon with your knuckles. If you hear a dull thud, the melon is ripe. A watermelon not ready for picking will have a "...tighter, ringing or hollow sound," states the College of Agricultural, Consumer and Environmental Sciences at New Mexico State University.
Press the base of the stem with your thumb. If the stem slides off the melon easily, that's another sign your seedless watermelon is ready for picking.