How can i tell if my glucose monitor is inaccurate?

Calibrate: The glucose monitor company can send you solutions of a known glucose level. When you test the solution, if the answer matches the known level, the machine is accurate.

Calibrate, compare: Hi. You can use the calibration solution that comes with the meter, and/or you can compare it with the meter at your doctor's office. If you have multiple meters (common for type 1's; not so for type 2's), you can compare them.

Compare fingerstick: You can compare the CGM reading to your fingerstick blood sugar. Keep in mind that there is generally a lag in the CGM readings relative to the blood glucose because the CGM is actually measuring the glucose in the interstitial tissue. In addition most CGM manufacturers will allow for an error of 10-15% in the CGM readings in either direction.