Answer: A phone app that determines when to add air to your bike's tires.
Question: What is a solution to a problem that does not exist?
Now, I dare say that none of us will ever reach the level of competence possessed by the Belgian gentleman pictured above. To bend in close, gaze upon the rounded form of a tire as if it were a Michelangelo sculpture, breath deep a fragrant bouquet as if the nose were perched over the rim of a wine glass, and be able to tell the psi of an over, or under, inflated hoop of stretched rubber. It is the stuff of legend and, probably, myth.
At half that competence level, an expert may flick the middle finger against a sidewall and judge the inflation based solely on sound, everything between a ringing ping and a dull thud, telling the expert everything needs to know about the tires' pressure.
A little further off that level is the pinch method - judging pressure by the amount of compression the tire gives between thumb and finger. With some careful observation and a little practice, nearly anyone can master this technique. Study long enough and pressure can be judged within a few psi above or below. With very little practice the method is capable of revealing when air should be added. The testing of the tires is the most basic of pre-ride rituals; I can't recall a single instance that a ride did not start with a test of my tires' squishiness. With that much opportunity, the method becomes very easy to master.
I though it rather humorous when I read the other day (on BikingInLA) about some new phone app that will (I assume by some scientific means) tell you when your tire needs a little air, or conversely (perhaps) when it has too much. If you have read here for a while you may have gathered that I am not much of a techno nut - you won't find me clamoring for the latest in electronic shifting, nor for disc brakes on my road bike, and certainly not for rear suspension on the road (as I also recently read about). At the same time, though, I am not a techno-reactionary either, I mean I was one of the first to buy into Campy's Ergopower levers back in the early '90s.
The whole "there is an app for that" thing seems to be a bit much at times. The point of all this rambling ride is that with just a little practice you can keep that $2.50 in your wallet, or purse - after all that is like a third of a glass of foamy brew at your favorite local ale house.