You put a salad plate in the freezer for awhile to get cold then plop a little of your hot jam or sauce on it and put it back in for about a min. it will jell up. If it turns out to be the constancy you are looking for you are done cooking if it is to runny you cook for a while longer.
When do you do this test? It must be before the jam is put in the jars?
Is there danger of cracking the plate?