The preserves will continue to thicken as they cool. But if you are impatient like me, you can use something called the spoon test. Place a clean metal spoon in the freezer (I like to stick mine down in the ice bin) before you get started. When the preserves come off the burner, remove the spoon from the freezer and dribble a few drops onto the cold spoon. This will give you a pretty good idea of how well your preserves will thicken.