You can perform one of the 3 types of ketone tests and if they are positive, it shows that you are in ketosis.

If you wish to rely on a non-diagnostic way to find out, usually people around you will comment that your breath is smelly, or you can smell a fruity smell in your breath. You may also find a metallic taste in your mouth, or have a fruity smell in your urine.

Generally, the breathalyser test is the most economical way to test frequently after you are keto-adapted.