This is the easiest test, but also the least reliable. Dip a wooden spoon into the jam mixture, hold it over the pan and turn it around a few times. Once the setting point has been reached the jam does not just drop off cleanly, but tends to run together to form a flake.
Crinkle or saucer test
This is far more reliable. Before you start making your jam put a saucer in the fridge so that it cools. When the jam approaches setting point drop a little jam on the saucer and leave it for a minute. Then push the jam with your fingertip. If the jam crinkles then it has reached setting point.
This is the most reliable method. Put a jam thermometer in whilst cooking the jam, and once it reaches 104.5 degrees Centigrade setting point will be reached… although be careful pectin rich fruits can set at a couple of degrees lower, so it is best use this in conjunction with the crinkle test.