Use the spoon test from Epicurious: Dip a cool metal spoon into the hot fruit. Immediately lift it out and away from the steam and turn it horizontally. At the beginning of the cooking process, the liquid will drip off in light, syrupy drops. Trying again a minute or two later, the drops will be heavier. The jam is done when the drops are very thick and two run together before falling off the spoon.