If you have a probe thermometer, check the temperature frequently and once the jam reaches 220°F (104°C), it is ready. Alternatively, you can use the freezer method. Place a couple of small plates in the freezer while you are making the jam. Once you get to the end of step 2 and a line of separation is visible on the bottom of the pot when you drag a spoon through the jam, remove the jam from the heat, grab a plate out of the freezer and plop a dollop of jam on it. Place it back in the freezer for 2 minutes. If it has formed a skin and wrinkles when prodded, it's ready. If it's still liquidy, return the jam to the heat and cook for a few minutes more, then test again.