Two of the most frequently used methods for testing "doneness" of jelly without added pectin are the temperature test and the spoon or sheet test.
Temperature Test – Take the temperature of the jelly with a candy or jelly thermometer. When done, the temperature of the jelly should be 220°F, 8°F above the boiling point of water, if you are at sea level. NOTE: For each 1000 feet of altitude above sea level, subtract 2 degrees F. For instance, at 1,000 feet of altitude, the jelly is done at 218°F; at 2,000 feet, 216°F, etc. See Table 1 below.
For an accurate thermometer reading, place the thermometer in a vertical position and read at eye level. The bulb of the thermometer must be completely covered with the jelly but must not touch the bottom of the saucepot. (Remember to test the accuracy of the thermometer by placing it in boiling water.)
Spoon or Sheet Test – Dip a cool metal spoon into the boiling jelly mixture and lift the spoon out of the steam so the syrup runs off the side. When the mixture first starts to boil, the drops will be light and syrupy. As the syrup continues to boil, the drops will become heavier and will drop off the spoon two at a time. When the two drops form together and "sheet" off the spoon, the jellying point has been reached.
from from: http://www.uga.edu/nchfp/
