It is useful to measure ketones because to know for sure you are in ketosis. Measuring can also let you know how your body reacts to different types of food or other variables such as exercise or ketone supplements. Also, when you are new to the ketogenic diet, it can be very beneficial to measure your ketone levels. However, it is possible to estimate when you are in ketosis using physical signs displayed by the body including increased thirst and urination, improved mental clarity and reduced appetite.
Once individuals are keto-adapted it is still useful to measure ketones occasionally to confirm optimal ketosis is being achieved. Optimum ketone levels vary from person to person and depend on individual goals. Ketones can be found in the bodies of diabetics and non-diabetics alike.