This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
From: Jean P. Nance
How to reach the jelling stage/The Fork Test
There are a couple of other tests for "jelling". One is "when it sheets from
a spoon", but I have found this confusing and sometimes deceptive. My
favourite is "when it closes the tines of a fork". I have found that it really
should be a silver plate fork, not stainless steel. Dip the fork in, bring
it out and observe. If the mixture stays in a sheet between some of the
tines, the jam is pretty near done. I usually cook it a few more minutes
just to be sure. At times my jam is a little stiffer than some people would
like, but better that than runny. Experiment to see how much "closing" means
jam is at the stage you like.