Does it depend on the category of watermelon? The watermelons in my garden have stripes down their body.
How Do I Know When a Watermelon Is Ready to Be Harvested
Certified GKH Gardening Expert
If the tendril is green, you should wait to pick the melon. Harvest when the curled tendril closest to the melon on the vine begins to shrivel and dry up. Also, give it a thump and see if it sounds hollow. If not, leave it for another week or so and try again for either the hollow sound or the tendrils dying. Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/fruits/watermelon/pick-a-watermelon.htm
I agree with the above post thats how I have done it for 3 years.