Q: I have a hard time telling when my watermelons are ripe. They have been the size of a cantaloupe for several weeks.
A: Watermelons can be slow to fully ripen because they depend so much on the right ratio of leaves to fruit. Leaves make energy that the fruit turns into sugar. If there aren’t enough leaves to serve all of the fruit, none of the fruit grows large or ripens fully. The exact ratio is loosely defined, but my guess is that you need to remove some of the fruit from your vine. Examine the vine to find the small tendril across from the fruit stem.
When it turns yellow or brown, the watermelon is ripe. More details at When to Harvest Fruits and Vegetables