You could tell when one tire looks lower than the other ones. Most of the time, a good quality tire will hold the same pressure for at least 3-6 months or possibly more. Temperature will cause it to contract & expand, so there is a little variance whenever you have a temperature (climate) swing or a road trip. On an average, most people add a few pounds to it about once or twice a year. The other option is to get an aftermarket TPMS system to monitor it everytime you drive. Al, did you look into using nitrogen instead of regular compressed air?