The last tendril should be dried and the spot on the bottom should be white, not yellow... When I picked my cantaloupe they literally fell off the vine. Watermelon does not seem to ripen up, on the counter- so be sure. Good luck- enjoy your harvest!
Although there are several indicators, I've found the dried tendril closest to the melon to be the easiest and best way to tell. It should be very brown and dry.