So you are at the grocery store and you have no idea which watermelon to choose, these tips might help and they are easy as 1, 2, 3 ......
1. LOOK – Your watermelon should be firm, symmetrical and free of major bruises or scars. Some minor scratches are okay, however. After all, the purpose of that thick rind is to protect the delicious contents inside. The outer rinds of ripe watermelons should also be dark green in color.
2. LIFT – The ripest watermelons have the most water. And since watermelons are 92 percent water, your watermelon should be relatively heavy for its size.
3. TURN – Turn your watermelon over and check out its bottom, which should have a creamy yellow spot (also called “the ground spot” and seen in the image above). This is where the watermelon rested on the ground while it soaked up the sun on the farm. If this spot is white or greenish, your watermelon may have been picked too soon and might not be as ripe as it should be.
A great blog all about watermelon is "What About Watermelon"