Start by placing a small plate in the freezer before you begin making your jam. After your jam has cooked for at least 10 minutes and you think it's thickened enough, remove the plate from the freezer. Add a dollop of hot jam to the cold plate and tilt the plate so it's straight up and down. The jam will start to run, and if it moves slowly then the jam is set. If it slides quickly, continue cooking and test in another 3-5 minutes to see if it's set.