The simplest way is to use a thermometer to check the temperature. You will want the jam to reach 220°F. 220°F is the heat point at which the sugar bonds with the pectin and forms a gel. Remember that jam can take 24-48 hours to set up fully. So if your jam is still on the thinner side, don’t worry. Know that it can set up more as it sits in the fridge. (More info below on what to do if your jam never sets up.) Another method to test if the jam is ready is the “plate in the freezer” method. (Remove jam from heat while conducting the test.) Place a small plate in the freezer when you begin cooking your peach jam. When you’re ready to test to see if the jam will set, remove the plate from the freezer and place a quarter size amount of the jam in the center of the plate. Place the plate back in the freezer for one minute. After a minute, run your finger through the jam on the plate. If it wrinkles and is gel-like, the jam is ready. If the jam “fills in” the line you made with your finger, you should cook the jam for another few minutes.