First two melons came off the vine today, August 21: a 25.6 lb Orangeglo and an 18.8 lb Strawberry. Won’t have time to crack into them today, but wanted to take a moment to illustrate two things often talked about in terms of judging ripeness: field spots and sugar spots. Not all 'melons will display field spots under all conditions—the Orangeglo did not have one—but many 'melons do, and Strawberry is very reliable in this regard. Generally, a ripe Strawberry will have a nice cream-to-yellow field spot where the melon was resting on the ground. Frequently, a ripe and often very sweet watermelon will show what I (and presumably others—if we’re indeed talking about the same thing!) call “sugar spots”----tiny black dots on and around the field spot. These are sometimes scattered in appearance, at other times they are found in concentrated groupings (as illustrated below). They seem to be fungal in nature and presumably form where sugar has leached from the bottom of the watermelon. 'Melons that display such spots are, in my experience, usually ripe and good-tasting. Here is a pic of my Strawberry’s field spot (it has a more yellow cast in person) and a close-up of said sugar spots.
My initial indicator is always the tendril nearest the melon. If completely dead, I check for a field spot/sugar spots. I also thump the melon and listen (as another grower once explained) for a resonance similar to smartly thumping one’s chest—a sound neither too sharp (under-ripe) nor too dull (over-ripe). Since it lacked a field spot (or noticeable sugar spots; and I have seen them without a field spot) I judged the Orangeglo on the basis of tendrils and sound.
Now, I may cut into these two melons in the next couple of days and make a fool of myself 
. . . . but, in most cases, I have found these sound indicators of ripeness.
The Strawberry, as you may have noticed, is a little irregular in shape. As discussed above, this seems a quirk of the cultivar—and the only minor fault that (thus far!) I’ve found in it.