Lipolysis, the process during which your body primarily burns fat as fuel rather than glucose, causes ketosis, which in turn generates ketones, the by-products of fat breakdown. During ketosis, ketones are released in your breath and your urine.
It usually takes at least two or more days of eating very low carb to reach this stage. Some people say they have a metallic taste in their mouth or their breath with smell slightly different (not bad almost sweeter). Mostly people feel energetic, their appetite becomes somewhat suppressed and they will often be thirsty.
Apparently there are actual litmus papers you can buy at some pharmacies that will detect ketones in your urine but I have never bothered with that sort of thing.