Everybody has their favorite secret for picking out great watermelons! My three top tips are to look for a large yellow spot on one side of the watermelon which can indicate it was sitting in the field ripening for a good long time, pat or knock on the outside of the watermelon to listen for a hollow sound which indicates good flavor, and look for "bee stings" where the skin of the watermelon is puckered or looks like it has vein-like thin brownish lines on the outside, which supposedly shows that the watermelon has good flavor. Here's an article with even more ideas on how to pick a good watermelon!