Are there alternative ways to test fruit preserves for doneness? 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?
Have you tried using a candy thermometer and testing the jam when it's at 220°F?

An alternative to the method that you use is to use a spoon and do the 'two drop test'. If you dip a cold metal spoon into your jam mix and then lift it. When the mixture is only just boiling it will drip off and be light. As the mixture continues to heat the drops that fall from the spoon will be heavier  When the two drops form and fall off the spoon it should be ready. I'll be honest though I much prefer the method you use and this is just an alternative.