Purchase a calibrated standard from the National Institute of Standards and Technology or a comparable organization if you are outside the United States.
Leave the standard in an accessible place before taking any measurements. In a factory or laboratory setting, this may be in the inspection area or at an operator's table, while the standard may be brought along in a protective portable case for field work.
Measure the standard using the same tool being used to take readings.
Compare the readings on the testing equipment against the standard's calibrated dimension. NIST traceable standards, or any standards calibrated by a third-party calibration service, will be accompanied by paperwork showing the measured value of the standard. A calibrated 100 volt standard being measured by testing equipment with an accuracy range of 0.1 percent should read from 99.9 volts to 100.1 volts if it is functioning correctly.
Discontinue using the testing equipment if it will not match the calibrated standard's readings.