 Belly Sugar Farms co-owner Myron Franklin says the first two things to look for are a creamy yellow spot and a non-shiny dull exterior. That indicates proper growth. Franklin says the third thing to do when choosing a watermelon is the old thump test. It'll have a nice kind of sharp sound to it when you whack it, you know. I usually like to slap it myself instead of knocking on it. But if that sound is a little more sharp as opposed to a dull sort of sound, like a hollow sound,