You may be able to tell that you're in ketosis by a change in your breath (many report halitosis, or bad breath, due to higher levels of the ketone acetone coursing through the body). Others experience the keto flu for anywhere from one day to two weeks as the body cranks up the ketones. Symptoms of the keto flu are very similar to the influenza virus, including nausea, fatigue, vomiting and diarrhea. Hydrating well and starting slowly can reduce your risk for this.
At the core of the classic keto diet is severely restricting intake of all or most foods with sugar and starch (carbohydrates). These foods are broken down into sugar (insulin and glucose) in our blood once we eat them, and if these levels become too high, extra calories are much more easily stored as body fat and results in unwanted weight gain. However, when glucose levels are cut off due to low-carb intake, the body starts to burn fat instead and produces ketones that can be measured in the blood (using urine strips, for example).