Source: Martina. "Measuring Ketones." Web blog post. KetoDietApp.com. Compumaster Ltd., 30 Nov. 2013. Web. 14 Apr. 2016. <http://ketodietapp.com/Blog/post/2013/11/30/Ketosis-Measuring-Ketones>.
There are three types of ketone bodies: Acetone, Acetoacetate and Beta-Hydroxybutryate (BHB). Unlike protein intake, where the daily amount is determined by your lean mass weight, the amount of carbs to enter ketosis can be estimated using:
1) Blood ketone meter
It's the most accurate way to measure ketone bodies, beta-bydroxybutryate (BHB, although not technically a ketone body). Blood ketone meters can precisely determine the level of ketones in your blood but they are also pricey. To give you an idea, the meter itself is about $40 and the test strips cost $5 each. If you want to test yourself daily, it will cost you $150 per month plus the cost of the meter. I don't know about you but I'd rather spend it on quality food or a gym membership. On the other hand, if you are on the Restricted Ketogenic Diet and numbers matter to you, a blood ketone meter may be the best and only option you have.
2) Breath testing - "Breathalyzer"
According to Volek and Phinney in The Art and Science of Low-Carbohydrate Living (chapter 13, Ketones - To Measure or Not), a non-invasive and cheap alternative is to measure breath acetone concentration. Acetone is one of the ketone bodies that results from a break down of acetoacetate.
The Ketonix Acetone Breathalyzer is available and offers an easy and inexpensive way to test your breath ketones (acetone). Keep in mind that breath ketones do not always exactly correlate with blood ketones and are affected by several factors (alcohol consumption and water intake). You can find my review of the breathalyzer here.
3) Urine ketone strips
Ketostix, Uriscan and other urine detection strips are not as accurate and may not work for some people. They only show excess ketone bodies excreted via urine (acetoacetate) but tell you nothing about the level of ketones in your bloodstream (BHB) which is most probably higher.
Some people use them to test if they are sensitive to certain foods that may be keto-friendly but still have a negative effect on their weight loss. They are easy to use and fairly cheap. You'll pay about $10 for 50 strips, that's $6 a month if you test yourself daily. If urine detection strips don't work for you, use one of the other two methods.
Listening to your body's signals is another way of finding out whether you are in ketosis. When your body is in ketosis, you may smell of acetone. This could be sensed in your breath, sweat or urine. Some people refer to this as ketogenic "fruity" smelling breath. If you detect any of these signs, you are more than likely in ketosis.