Harvesting with friends and family is awesome.
My nephew Nathan found a watermelon that looked ready to pick. We showed him the test to see if it's ready:
Tap on it, and it should sound like a dull thump.
It's important not to pick it too early. It will only ripen on the vine, much like citrus on a tree. If picked too soon, it will soften, but will never get any sweeter than it would have on the vine.
We ended up giving the melon to our neighbors, who said it was great. I have to say that sharing the harvest is even more satisfying than enjoying it myself.