When your body switches its primary fuel source from carbs to fat (will take 2-3 days the first time) your body will begin producing ketones to break down the fat. One sugary salad dressing or a piece of fruit can kick you out of ketosis.
So, how do you know if you are in ketosis? Keto Strips are pee strips that you can buy at any local pharmacy or online at Amazon. Simply urinate on the strip, and if the color darkens, then your body is producing ketones. The darker the color the better.