The watermelons are going great and we just wanted to let you in on a tip that we read about and have found to work well. The ideal way to determine when to harvest watermelon is to record when its bloom flowered and then count the days depending on the variety being grown. The vines don't produce all of the blooms at one time and few people are going to do this. A tried and true method when growing watermelon on the ground is to harvest when the underside turns yellow. Unfortunately this can't be done when the vines are growing on a trellis, because the fruits are hanging in midair! Many also claim to be able to thump the fruit and know by the pitch when it is ripe, but this isn't 100%.
Dead tendril indicating time to harvest
For the tone deaf people that don't record every blossom, we have a solution. Keep an eye on the tendril that comes out of the vine where the melon's stem joins. When this tendril dies, harvest the melon. This has worked like a charm for us. This tendril can die back overnight, so keep an eye on it when the melons are starting to ripen.
Here's the link to where we got this tip. It is a phenomenal resource for those wanting to grow melons.