Please take a look, and let me know if these watermelons are ready for harvest... I've gotten so many contradictory methods that now I'm confused (not that that's hard to do).
Here is a link that might be useful: DFW Gardener
Hard to tell. Have they been in the ground 3 months? I believe the maturity date is about 90 days from setting out. Any roadside stands selling watermelons in your area? If you have one you can spare just try it. If you're not sure, wait 2 weeks. Just my opinion.
Best 3 indicators I know of:
1) Brown stem. The stem going into the watermelon itself starts turning brown. Not the ones next to it, the actual stem itself.
2) Yellow belly. Pick it up and look at the bottom part that touches the ground. Has it turned yellow?
3) Matte finish. This one is harder to learn to spot, the fastest way to tell at a glance. While growing, watermelon tend to have a certain almost satin finish to the melons. When they're ripe, the satin finish turns more matte. This one is one you just have to see for yourself over time though.
No single indicator is 100%, but the more of them you see, the more assured you can be.
Keep an eye on the tendril the little curley stem that sticks out across from where the watermelon stem joins the vine. Once it dies wait five days and you can pull the melon and any other melon in the patch that has a dead tendril. If you do not wait the five days when the first tendril dies you will pull some half green melons. I grow 15 acres a year and that is how we pull every melon in the patch.