Getting the amount of sugar right in your jam or jelly is important for proper setting up. Too little sugar will turn into a runny mess.

Using a Thermometer

An accurate thermometer provides the simplest and safest way to tell when the sugar has reached the proper concentration. Star by measuring the exact temperature at which the mixture first boils (it will vary depending on weather as well as altitude, so take a new reading each time you make a batch of jelly). As cooking progresses and water boils away, the sugar concentration will rise and the temperature goes up. When the thermometer registers 8F to 10F above the initial boiling point, the jelly is done. 

Using a Metal Spoon

As an additional test, dip a cold metal spoon into the mixture and hold the spoon away from the heat. If the jelly runs off in a sheet rather than individual drops, it is ready. 

Using the Freezer

A third test is to put a spoonful of jelly on a plate and put it in a freezer. If the sample becomes firm after one or two minutes, the jelly is ready. The freezer test can also be used for jams and preserves, the sheet test using the spoon cannot.