Most manufacturers now list the cacao content of chocolate on the wrapper.
The percentage of chocolate liquor, or cacao mass, listed on some bars as the percentage of cacao, which is how I refer to it, indicates the amount of cocoa solids and cocoa butter in the chocolate. Besides the cocoa solids and butter, the rest of the chocolate is mostly sugar. The percentage of cacao in the chocolate, however, does not necessarily indicate its degree of bitterness. Flavor also comes from the variety of beans used and the methods of production. The percentage of cacao in bittersweet, semisweet, or milk chocolate, though, does determine consistency of the chocolate. The higher the cacao percentage, the more cocoa solids and cocoa butter, which are solid at room temperature. Chocolate that has a higher percentage of cacao will make a cake crumb or frosting more firm as well as more chocolaty. By merely exchanging the same quantity or weight of chocolate in a recipe with a chocolate of a different cacao percentage, you may either be adding more cacao and less sugar, or using less cacao and adding more sugar.