For cantaloupe, it will have a sweet aroma, the green colour will shift to beige, and the stem will easily separate from the melon. Honeydew melons will shift in colour from green to creamy yellow and the end of the melon opposite from the stem will have a slight give when pressed. For watermelon, the 'belly' spot where the melon was resting on the ground will shift from white to having a yellow-ish tint.