I've worked in Produce distribution for almost 30 years and I've also gardened.  There is no fool-proof way of telling how ripe a watermelon is until you cut it.  Thumping it only tells you how juicy it is and you have to know what to listen for.

One Indicator: When the vine closest to the melon starts turning dark and drying up, it's ready for picking.

Seeded/seedless; which is sweeter?  Depends on the variety and growing conditions.