Has the vine where it meets the fruit dried out? That's when you can judge ripeness. I believe you can even leave them on the ground a few days to improve. The sugar develops at room temperature so don't put them in the frig immediately.
Some people confuse flavor and sweetness. If they are lacking flavor it's probably because they were overwatered.
(SIDE NOTE: Or thy did not receive the right amount of sunlight during the growing season. -Birdymckee)