Ketosis is a metabolic state that your body can enter when you're either fasting, eating a low-carb diet, or consuming exogenous ketones. In this state, your body burns ketones as well as fats and carbohydrates. Learn more here: https://hvmn.com/blog/ketosis/what-is-ketosis
Being in ketosis means that you have elevated levels of ketones in your blood. The best way to know if you are in ketosis is to measure the levels of ketones (BHB) in your blood. You can also estimate blood levels by measuring ketones in your breath or urine.
No ketosis: under 0.5 mM BHB in blood
Low ketosis: 0.5 - 1.5 mM BHB in blood
Moderate ketosis: 1.5 - 3 mM BHB in blood
High ketosis: over 3 mM BHB in blood