Originally Posted by
alaskanb3arcub
I have seen valve caps rated for a certain PSI(35 or 60, iirc) that would give a quick visual indication(green/yellow/red depending on the pressure in a tire). Can't remember where though, and not really up to searching for them at the moment.
Those have been on the market for quite a while. There are some catches. They're designed for tires that take a large volume of air (automotive tires) and actually require that the tires are toped off more frequently because they do leak a bit of air. Also automobiles generally run lower pressures and still its recommended to check them every week or every day if driving on the highway. A high pressure low volume bicycle tire? You might not finish the ride!