Watermelons that are allowed to ripen on the vine have the best flavor, so if possible protect plants from light frosts.
Check under the melon: if it has a yellow or light colored bottom, it should be ripe. If its stripes are found all around the melon, it’s just not ready yet.
If you have a good ear, tap on the watermelon: if it sounds hollow, its ripe. Not hollow, unripe.