There are three primary ways that ketones can be measured. Through urine strips, blood ketone meters, or breath ketone testing.
Each of these three techniques has their own pros and cons; however, blood testing is considered to be the the most reliable. Keep in mind that even results from this testing technique can be challenging to interpret and when and how you test plays a major role in your results.
If you are going to test ketones, it is recommended that you establish a baseline by limiting as many factors as you can. This can be done by testing in the morning, a couple hours after waking up, prior to caffeine or food consumption, and before exercise.