At a number of RV events I have tested different gauges against a digital 0.5 psi reading gauge that has been checked against QS9000 ISO/IEC 17025 approved master laboratory pressure gauge and found accurate to +/- 0.5psi. We have found that about 15% of the tested gauges were wrong by more than 5psi with a few off by more than 12psi. In general the most accurate were digital. The least accurate were the "stick" gauges with the sliding scale.
I have suggested that if you are accurate to within 5% of your goal cold tire inflation you are probably OK but obviously more accurate is better. The 5% tolerance is based on an assumption that you have added 10% to the minimum required per measurement of actual tire load and consultation with your tire companies Load/Inflation tables. It has also been suggested that using any gauge is better than not checking your tires at all.
Now how to know the accuracy of your gauge in day to day use. I suggest the following based on personal observation (no sponsorship or compensation received).
Get two digital gauges. One can be like Accutire MS-4021B as seen on the right
I have hose extenders as seen below
My master is packed away in a box so it does not get damaged or knocked around. Once a month I measure one front tire with both gauges and record the readings. The difference between the two readings should not change by more then 0.5psi from new. If I notice a change in tire inflation not explained by significant temperature change or puncture I will compare the daily gauge against the master gauge. I figure the chance of both gauges going bad the same amount and in the same direction at the same time is vanishingly small. The 4012 is available at $10 and occasionally for less than $7.
TPMS readings have not found to be as accurate as hand gauges. Their purpose is to be a warning of a loss in pressure. You should set tire pressure with the hand gauge. You should see a fairly constant difference between the TPMS and the hand gauge.
It is important to measure your tires when they are at ambient temperature. This means the air temperature. NOT exposed to the sun or even under a tire cover or having been driven on for at least two hours.
Tire pressure changes with temperature and you can use the rule of thumb of 2% for every 20F.