VISUAL CUES: Before you run a jam test, watch the bubbles on the surface. They get bigger as the jam approaches its gel stage. They start out small and profuse, but become less numerous as time progresses.
TEMPERATURE: Use a candy thermometer and When it nearly reaches 220°F start to look for these other visual cues to confirm that it's reached the gel stage.
SHEETING: Stir the mixture with a wooden spoon. Lift the spoon from the liquid so that it's horizontal to the pot. Gently tip the spoon (over the pot) towards you. The liquid will run off the spoon at first and then in individual drops. When two drops come together on the spoon and become one big drop, that's sheeting and an indicator that the marmalade has reached gel stage.
WRINKLE: This is probably the most common method for determining if you've reached the right consistency - but you have to be prepared for it by placing a small plate in the freezer ahead of time. When you want to test the marmalade, place a small spoonful of the marmalade on the frozen plate. Let it cool. Use your index finger and place it flat on the plate beside the pool of jam. Run your finger on the plate sideways into the jam. If the jam starts to wrinkle like a Shar Pei puppy, it's ready.