Undoubtedly, making jams, jellies and preserves is best with a reliable thermometer. However, there are other ways to ensure the recipe is working and the preserves will set.  Before you start, place a couple of small plates or saucers in the freezer for testing the consistency of the preserves. When ready to test, spoon a sample onto the frozen saucer: if it's thick enough to stay mounded without running or spreading, the preserves are ready. If it's too runny, return the pot to a boil and cook for about 5 minutes more before testing again.  Jam is a thick spread made from chopped, crushed, sliced, or puréed fruit and sugar. It can be made with or without commercial pectin. Jelly is made from juice only. It can be made with or without commercial pectin. Preserves are another thick fruit spread made from fruit cooked with sugar, but in this case, large pieces of the fruit, or the whole fruit (as in the case of berries), are suspended in a firm jelly or less-gelled syrupy base. Marmalade is a soft spread made from the peel and pulp of citrus fruit and are used as spreads and glazes.