The only 100% definite way to know is if you use something called ketostix. You can get them at the chemist or on amazon. They're little strips that you test your urine with...provided you show any colour, you are in ketosis. Signs of ketosis though are the kinda bad taste you get in your mouth, sorta yucky breath etc....I find those eventually go away though. Thankfully. Essentially if you have been following it, as I understand it, within about 4 to 7 days you will likely be in ketosis. I hope that helps a little!