How can you be sure to choose a sweet watermelon?
Best way would be to make friends with some farmers, and then get your melons from them :D Works for me, my friend Tony grows the best watermelon I've ever had, and it's organic to boot.
if it sounds hollow when you knock on it with your knuckle
They make a nice hollow thump :)Actually, I don't have much luck. A lot of times I'll buy a melon sliced in half. That way I can see the fruit inside.
Creamy yellow bottom
Gently knock on the watermelon. If it is hollow that means it is very likely to be sweet and juicy. :)