This article is relevant to non-diabetics and does not relate to diabetic keto-acidosis.

Ketones can be measured in urine, breath and blood.

Urine testing is the most common and cheapest method of testing for ketones. It becomes less reliable as a method of establishing nutritional ketosis over time as the body becomes more efficient at consuming ketones for energy.

Measuring ketones in breath correlates reasonable well with blood ketones. The initial outlay is high but there are no ongoing costs. Correct technique is important. https://www.ketonix.com.au/pages/what-is-the-ketonix

Blood ketones are the most accurate measure of nutritional ketosis and although the testing device may be a reasonable outlay the testing sticks are an average of $9.00 for a box of 10. The same device can also be used to test blood glucose levels. https://myfreestyle.com.au/products/freestyle-optium-neo-blood-glucose-ketone-monitoring-system/