The quality is highly dependent on the high total sugar content.   Other factors determining high quality are deep red and pleasant texture of edible flesh.

Maturity and size are essential factors in watermelon property.  The larger melons have more edible flesh, proportionately to weight, than smaller ones.  Maturity of watermelons is difficult to determine without plugging and testing.  Ripe melons of good quality are firm symmetrical, fresh,  with good characteristic color for their variety.  The lower side of field raised melons should be yellow where the melon came in contact with the soil.  If the melon is very hard and is white or pale green on the underside, it probably is immature.  If so, the total sugar content will not increase after it comes off the vine.  the old wives tale of thumping the melon providers exercise but little else in determining the ripeness of the melon.  Melons should have a good red flesh, that is crisp, and not mealy or water soaked, from bruising.