Pectin is a kind of natural gum which is found in the cell walls of fruit and is vital for a good set. Simmering fruit gently helps to extract pectin. If you want to test your preserve for pectin content do it after cooking the fruit and before adding the sugar. Or before straining if making a jelly.
Take a teaspoon of the liquid in the pan and put it in a teacup. Let it cool for a moment, then add 3 teaspoons methylated spirit and shake gently together. If the juice forms a single clot the preserve will set perfectly. Several small clots means the mixture needs a little longer simmering to get a more concentrated juice. Discard test sample.