Here in watermelon country they look at the pattern of the melon on the stem end. When it begins to break up and look like lightening bolts, it is time to pick.
You also want the bottom of the melon to turn a nice yellow color as opposed to white.
I am sure it is hard to resist picking and sampling but hoping they are ready before your frost. Kristi
edited to add: my apologies, I am NOT a watermelon expert unless eatin' them counts.
