In the absence of carbohydrate, and after fasting, the liver will produce ketone bodies (plus glucose, via gluconeogenesis, of course). The ketone bodies do have a distinctive smell, which can be detected in the breath. The urine will also smell of them (if the other urine smells don't overwhelm them).

The smell is difficult to describe . Some people cannot smell acetoacetic acid as such, but can recognise the acetone smell in the breath. If you are able to smell acetoacetic acid it is very distinctive, and not easily mistaken for anything else. The closest other smell resembling that of acetoacetic acid is very ripe apples, but not everyone finds that a useful description.