So I looked it up on how to find a ripe watermelon for you and this is what Google said:

"To find a ripe watermelon, look for the field spot, which is a large, yellow patch on one side of the melon.

The color should be creamy, almost butter-like yellow, and the bigger the yellow belly, the more time the melon spent ripening on the vine.

The green stripes should be a deep, dark green, while the pale stripes should be a creamy, light yellow.

A ripe watermelon will feel sturdy and have a resonant, satisfying hollow sound, which means it's brimming with juice and at its peak of ripeness.

If the thwap is dull sounding or the fruit feels soft, pass on it. If you start lifting watermelons, you should find that their weights are similar, and a dried tail indicates that the watermelon is ripe."

So the next time you go hunting for watermelons, look for some yellow patches! I hope you get the chance to use this information one day! Have a nice day! :)