There are three methods:
TEMPERATURE TEST: Before cooking the jelly, take the temperature of boiling water with a jelly, candy, or dee-fat thermometer. Cook the jelly mixture to a temperature 8F higher than the boiling point of water. At that point the concentration of sugar will be such that the mixture should form a satisfactory gel.
SPOON OR SHEET TEST. Dip a cool metal spoon in the boiling jelly mixture. Then raise it at least a foot above the kettle, out of the steam, and turn the spoon so the sirup runs off the side. If the sirup forms two drops that flow together and fall off the spoon as one sheet, the jelly should be done. This test has been widely used; however, it is not entirely dependable.
REFRIGERATOR TEST: Pour a small amount of boiling jelly on a cold plate, and put it in the freezing compartment of a refrigerator for a few minutes. If the mixture gels, it should be done. During this test, the jelly mixture should be removed from the heat.
SOURCE:How To Make Jellies, Jams, and Preserves At Home. United States Department of Agriculture. Extension Service. Superintendent of Documents, Washington, D.C. Home and Garden Bulletin Number 56.