How do you know if your jam, jelly or marmalade is done cooking? Use the freezer test!
Prior to cooking your confection (jam, jelly or marmalade), place a saucer with 5 metal spoons in your freezer.
After the recommended average cook time (based on your recipe) and visual indicators, test for doneness using the Freezer Test.
To test, remove the confection from the heat and carefully transfer a half-spoonful to one of your frozen spoons. Place the spoon back in the freezer for 3 to 4 minutes, then remove it and tilt the spoon vertically to see how quickly the jam, jelly or marmalade runs. If it runs slowly, and if it has thickened to a jammy consistency, it is done. If it runs very quickly or appears watery, cook it for another few minutes, stirring, then test again as needed with remaining chilled spoons.