I've tried twice to make fruit preserves using only the natural pectins in the fruit. The first time with plums where the jam was much to thick, and yesterday orange marmalade which came out tasting great but a bit on the runny side.
In both attempts I tested for doneness by letting a few drops of hot jam fall onto a cold plate and letting it cool before pushing it around with one finger to see if a crinkly skin has formed.
In my hands this test is clearly not fool-proof. Are there alternative techniques for those of us who have not yet developed an eye for crinkliness?