The curl on vine next to melon start turning brown that when I cut one open and check don't wait until hole curl brown. Most watermelon have maturity date from seed coming up like search watermelon maturity date Charleston Gray melons.

Fertilize just before melon starts to run late fertilizer make longer maturity date and unsweetened melons.

Fertilizing water melon best take corner hoe an dig V trench around plant use 1 pint of 10-10-10 thats pound per hill put in trench and cove with soil trench need to be about 3 inches deep this be last fertilize needed and vine grow and set many melons. Watermelons are dry land crop so water very lightly and 3 weeks in-between watering.

Practice by shopping for ripe melons at the market. Get good at it. A ripe watermelon will be heavy for its size. The "ground spot" will be cream colored, and won't be ripe if still green or white. Put the melon to your ear and tap the melon with your finger. The tap is performed by touching the tip of your first finger to the tip of your thumb, as if giving the "ok" sign, and then give a little flick of your finger. Hit the melon with the flat surface of your fingernail. The sound will be a dull "punk" when tapping with your finger. If you hear "pink", the melon is not yet ripe.

On the vine, check for the closest tendril to be dried up. There can sometimes be a little "spoon" leave that dries up as well. Splits in the vine leading up to the melon is a good sign as well. From there, followup with checking the ground spot and finger tap as you would at the market.