Ripe watermelons make a hollow noise when knocked on. Unless you are familiar with the sound, this can be a little difficult to pick up on. The first year I grew watermelons, I knocked on a melon at every farmstand and grocery store that I came upon, just so that I could get a memory for the sound. Then when I thought the melons might be getting ripe, I started knocking on them every day until I could hear a change in the sound.
The days to maturity printed on the seed packet can give some clues as to when to start checking for ripe melons. This is calculated from when the seeds germinate. It is based on the climate of wherever the seeds were grown, so obviously it is not very accurate and is more helpful for comparing varieties than actually predicting harvest. If everything seems to be a little ahead as it is here, then start checking a little early. Size is another clue to maturity; Sugar Baby watermelons are small melons about 11″ diameter I think. I hope this helps…