How to Test Jam for Doneness:1) The Temperature Test: The jam should be at 220F (this is the magic number if you’re at sea level, like me); a cheapie candy thermometer works fine here.
2) The Sheet Test: Dip a wooden spoon into the jam and hold it sideways over the pot; the jam should fall off in a sheet, or the last jam to drip off should hang off the spoon in a bulging drop but not fall. (You can see what this should look like in
this post.)
3) The Freezer Test: Freeze a plate, then put a little jam onto the frozen plate and put it back into the freezer for a few minutes; the jam should gel and not run.