I would say the easiest, fastest and cheapest way to detect if you have stinky armpits is by lifting up your arm to take a sniff off the armpit in question. Voila, if it stinks you got stinky armpits. But Japanese of course would never sniff armpits in public, so they have a detector for that. With a scale on display from 0 to 10 where 10 is the stinkiest.
From their product page:
The process only takes around ten seconds, during which time Tanita’s ES-100 device analyzes for smell-producing particulate matter. Since it is reading the intensity of smells, you can check not only for bad odors but also perfume or aftershave that is too strong.
Buy it here: Stinky Armpit Detector