Depends on the scanner, depends on the vehicle. Some vehicles have very complex TPMS systems that monitor not just air pressure, but temperature and battery life as well (Porsche for example). Some are very basic and are unable to even tell you which tire is low, just that one of them is lower than the others. But if you buy a scanner or have your local tire shop scan the sensors, if one of them isn't reading because of a dead battery it will be come evident. The scanner will "fail to communicate" to that sensor.