You have two options that can be used separately or together. The first one is to place a candy thermometer in the cooking jam and let it reach 220°Fahrenheit/150°Celsius. The second option is to take a cold plate and pour some hot jam on it, then turn the plate around. The hot jam will quickly cool down and show its true consistency. If the jam is set, it won't spread, but if it's still runny, you should continue cooking and try the test again. Another sign of a set jam is when the fruits look translucent or glossy.