Opt for ‘nice and fat’ watermelon over long and skinny ones, as the latter haven’t fully matured on the vine and might lack flavor.
Be cautious with heavily bruised melons, as they might be mushy at those spots. The field spot, the area where the melon rested on the ground, is another key indicator. A yellow field spot indicates the watermelon ripened in the field, while a white spot suggests it’s underripe.”
“When you thump a watermelon, listen for a nice ‘ping’ sound. A dull thud might indicate flaws such as overripeness or hollowness.
Remembering these tips is easy with the mnemonic device ‘Look, lift, and turn’—look for a good shape without major flaws, lift to feel its weight, and turn to check the field spot.
Source: FOOD & WINE