Purchase some potassium metabisulfite, potassium sulfite, sodium sulfite or sodium metabisulfite from your local brew store. Dump 50g into about 100ml of distilled water, shake it up a few times, let settle (not all the bisulfite will dissolve)... then pour off about 5ml of the clear saturated solution of bisulfite into a test tube. Add about 100-200µL (0.1-0.2mL) of suspected ketone. Put a cork on, shake it up... let it sit for an hour or two. The whole test tube should fill with shards of bisulfite crystals if it's good ketone. If it goes sort of like chicken soup goo then there's ketone, but it's impure and needs to be distilled again.