How to tell if a watermelon is ripe and good to eat

We have experimented with different ways of cutting and serving watermelon flawlessly, a fruit that accompanies us during the summer dinners and to pleasantly surprise guests. But when you go to buy it, do you know how to identify if it's a ripe and juicy one? There are simple and quick "tricks" to pick the right watermelon among all those we see at the greengrocer: on the outside they all look the same, but there are considerable differences.