There are several criteria to look for to find a ripe watermelon that is sweet and delicious. Watermelon will typically be the sweetest during the summer months of June-August. You can still find a sweet watermelon in May but you have to use super-sleuthing to pick just the right one. 1. Look for a watermelon that has a light creamy, roughly round area on one side of the watermelon. This is the spot where the watermelon sat on the ground while it matured. Look for one that is creamy yellow. 2. The stem of the watermelon should be brown and dried out. If the stem is green then it may mean that the watermelon was picked too early before it was ripe. 3. Thump the watermelon with your finger like you are shooting a marble. You can also knock on the watermelon. The melon should sound hollow. Check several so you can tell the difference. 4. Lift the watermelon. It should feel heavy, the heavier the better for ripeness.