The first thing you need to is look for a large yellow spot on the melon.
You'll also want to look for distinguished dark green and light green stripes. The darker the green the better.
Next, comes the sound of the melon. You want to actually knock on the melon. The sound you are looking for is a hollow sound.
Lastly, you will need to pick up the watermelon. It should feel very heavy. The heavier it is the more water it contains. If it's light in weight it will be very mealy and mushy.