The easiest way to check when a marmalade has reached setting point is with a sugar thermometer, the marmalade will reached the jam stage at 220F, 104C. You can also use the cold saucer test, or combine both methods to be certain of a good set. To carry out the cold saucer test put a saucer in the freezer to cool, when cool take it out and drop a teaspoon of jam on the saucer then place back in the fridge for a couple of minutes. Remove from the fridge and push the jam gently with your finger, if the surface of the jam wrinkles then the jam is ready to pot. It is advisable to turn off the heat while testing or you may over boil the marmalade.