I didn't notice the taste in the mouth or bad breath for about 3 weeks, but I was definitely in ketosis as I'd gotten some ketosticks and tested myself after about 4 days. As Nic says the real foolproof sign is when you suddenly feel like you've got loads of energy after the first few days of feeling deprived - your body has kicked into keto at that point and you're suddenly getting loads of energy from your stored fat.