Making Sure I'm Setting My Tire Pressure Correctly

The next question is how do we know our pressure gauges are accurate? I took my best one, four ears old, to visit my mechanic friend at his workplace where he keeps a quite new, high quality one. It said mine was reading 60 when the pressure was 75 psi. Turned out the best pressure gauge I had was a digital one I got at a dollar store. Next best was an old spring one I inherited from my dad and cleaned/lubricated. The three of them agreed last summer.