I agree with the webbing and orange spots. Idk about the others per se but if you hold it in one arm and spank it, pick the most wobbly feeling one. That one has the most water in it and it's going to be the best in the batch. As an added bonus, when you're standing in the store spanking all the watermelons you'll look Very Insane :D
Grainy watermelons are drying out because they're too ripe. In the store, pick the wobbly ones. Add home, keep them in the fridge or somewhere cool, DON'T store them in a bag, and eat them within a day or two or they'll become grainy.
That's how every parent knows if the watermelon is good or not, slap it!