To test a food for starch, you can add a few droplets of iodine to it, if the liquid changes to a blue/black color then starch is present. To test for sugar you can use Benedict's solution, which will also have a color change from blue to yellow/red/orange. Add Biuret Reagent solution to test for protein, the solution will turn a pink or purple color... Note that this may not improve the taste or color of your gingerbread biscuit though :)