If you are following a well tested recipe for sorbet, the sugar content should be correct; however, the ripeness/sweetness of fruit can vary, which could effect the recipe. Strive to use fully ripened fruit at its peak level of sweetness. If you want to be absolutely sure the sugar content is optimal, you can invest in a refractometer – a pro kitchen gadget that measures the concentration of sugar. The goal is to reach a concentration between 20% to 30%. Then there's the "egg trick" for measuring the sugar content of sorbet base made with strained fruit juice. It involves floating a raw, clean, fresh egg in the sorbet base. When it's at the optimal level of sweetness, the egg will float to the top and a bit of the eggshell (the size of a quarter coin) will be visible. Here's a great video of Chef Thomas Joseph showing how it's done.