This is the test I most often use to determine if my jam is ready. When you start to cook the jam, put a saucer or small plate in the freezer to chill.
When you think the jam may be ready, put a little dab of the hot stuff on the chilled saucer, and put it back in the freezer for one minute. Take your jam kettle off the heat while you do this so it doesn’t cook down further. When the minute is up, check the jam: when you push at the blob with your fingertip, do little wrinkles show on the surface? If so, it’s ready. If not, it needs to cook a bit more.