i've been making lots of jam lately -- mostly using christine ferber's book, which i love (but which is sort of scant on detailed directions). her jams don't use pectin, so you need to check the set to see when it's ready. i keep trying to do that sheet test with a spoon, but my jam thickens but never sheets. recently i made a batch of plum-vanilla and kept testing with a spoon and no sheeting until the point when i started to smell caramelizing at the bottom -- definitely too much cooking -- so i quickly took it off the stove. it set just fine when cool despite no sheeting. the next batch also never sheeted off a spoon, but i had a hunch it might be ready and had been cooking long enough, so i took it off, and it also set perfectly. the thing is i'd rather not rely on my hunches, since i'm not the most experienced jam maker yet. a foolproof test would be much more comforting. what gives -- why no sheeting? is my spoon not cold enough? something else? can someone walk me through what needs to be done exactly?