There are a couple ways to know when jam has reached its setting point. The first is to check for something called "sheeting". For this method simply dip a spoon into the hot jam and then remove it turning so the jam runs down the back of the spoon. The jam is set when you see the drops hang on to the spoon, rather than just flowing down into the pot. Another method is to pop a small plate into the freezer when you begin making the jam so it has time to get very cold. Then when you think the jam is ready simply spoon a little onto the cold plate and give a few seconds to set, then run you finger down the middle. If the jam immediately seeps back where you finger has been then it is not yet set. If the space your finger made remains clear then the jam is set.