Right at the beginning I had a bottle of 50 small tablets (not the big glucose test ones) on which I could deposit a drop of one of my bodies precious fluids (the golden one) and it would turn blueish if it got a whiff of acetone.
Many years later I learned I could breathe in the general direction of my wife and she could tell me, but I’m a T1D so I never ate pear drops. These days I have an alert called “ketone” set at 250mg/dl and I have tested it against probably fairly accurate test strips and it is perfect; no false positives, no false negatives.
In all my T1D life (49 years) I’ve not checked for ketones more than 10 times, mostly recently while verifying the Dexcom G6. It’s like walking into the middle of the interstate then checking for oncoming traffic.