I knew someone used to knock on them with his knuckles. Told me that he could "hear" the watermelon be ripe. With that, he always picked out the ripest, sweetest, juiciest ones every single time. He spent quite a bit of trying to teach me, but I could never figure it out.
Thing was, he was deaf. Even though "hearing" watermelons was an impossibility, he swore up and down that it can be and is done that way.

Good luck on your crop. I missed my planting this year.
