How I tell when jam will set

This is the method I use to tell whether jam will set (I find eating some of it with a spoon helps too...)
Before it is ready to set it will run smoothly off the spoon in a single stream when you lift the spoon out of the proto jam

When it has reached setting point it runs of in a number of streams side by side. All of which run slower (so it was hard to get a photo - also my trusty photographer got bored and I had to take the second one myself which was more difficult)