The best way to tell if your jam is done is to use a candy thermometer. Once the jam reaches 220°F, it can be removed from the heat. As it cools, it will set to a jam-like consistency. If you don't have a thermometer, you can use the frozen plate method. To use this method, just put a plate in the freezer when you begin making your jam. When you believe your jam is ready, remove it from the heat and spoon a little bit onto the frozen plate. Wait a minute or two before sliding your finger through the jam. If the jam wrinkles and has a gel-like consistency, it is done. If not, place the plate back into the freezer and test again in a few minutes.