ripe watermelons are ripe when you knock and they sound hollow because they are hollow because they are ripe. and the riper they are, the higher the water content thus the heavier the melon. a yellow dot marks the spot where the melon fell onto the ground and chilled until ripe as opposed to being prematurely picked. maybe find one thats nice and dark in green. mines stripy which perhaps is not the best but it was the last one there and i figured i could juice it which is an idea if you happen to pick a bland baddie.