There are many ways to test whether jam is ready. Edon Waycott prefers this method: Remove 2 tablespoons to a small saucer and place it in the freezer for 5 minutes. Meanwhile turn off the heat under the pan. When the test amount is cool, it will wrinkle slightly when slowly pushed together with your finger. If it doesn't, continue cooking for another 5 minutes and test again. When ready, the jam will be thick but will still flow from a spoon.
You can also test the jam by scooping some out in a metal spoon and then pouring it back into the pot. When the jam begins to pour out in a single sheet rather than in several different streams, it's done.