The best, and most accurate way to currently test for ketones is with a blood ketometer. 
They are available at the testing units themselves are quite cheap but the strips are around $3 per strip. Typically people test once per day, first thing in the morning. Anything over 0.5mmol/L is considered functional, nutritional ketosis.  

If you want to find out more about ketosis and diet consider signing up for the HPN level 1 certification: