I think that they will slip off the vine easily when they are ready. But, I'm not certain, it has been a long time since last August when I harvested the 3 I had ;-) .
my mother always told me there was three ways to know when a watermelon is ripe...the side that was on the ground should be a creamy yellow color...the rind should look dull instead of looking shiny...and the nearest tendril is dried out...when you find all of them signs, then the watermelon should be ripe