They should come with a calibration certificate from the manufacturer?
If not, you probably need to calbrate in the lab against a known good standard - most likely on a piece of material of known characteristics on a standard test machine in current calibration.
If they're use once stick-on units, you need to test enough to give you a statistically valid sample, identify the (probably) mean and SD characteristics and put 3-sigma error bars on your in situe test data. Better still, the strain gauge supplier did that and gives you the data.
P