How do I pick a watermelon at the supermarket?
I don't believe there is a fool-proof way to determine 'ripeness' without taking a slice out of it. The best you can do is look for certain signs:


Ripe melons have a hollow sound when you tap or slap the outside
Look for the patch where the melon would have been on the ground (called the field spot). If it's a yellow colour its probably ripe, if it's white, it's probably not.
It should feel relatively heavy when lifted
Weird areas on the skin aren't necessarily bad. insects may have tried to start eating the fruit because it is ripe, but have only marred the surface.


Unfortunately, melons don't continue to ripen once picked, unlike fruits such as apples, bananas etc. which contain ethylene. As a tip don't store melons with these kinds of fruit, they may well go 'soggy'.

Extra: NYtimes video on picking the right watermelon.