Choosing a Ripe Melon
There are two reliable methods:
Examine the underside of a melon. Ripe ones will have a cream or yellow belly, unripe ones will be white or green.
Ripe melons will sound hollow when you slap them. But what does "hollow" sound like?
Practice on your body. Give your forehead a couple of light slaps. Repeat on your chest. Repeat on your stomach. Notice how the sound changes for each one. The sound your stomach makes when slapped is the "ripe" sound you're looking for.
It's best to compare three watermelons side by side. You'll easily be able to pick the one that sounds most hollow when slapped.