First let me state that there are 2 parts. The first is to make sure that the data fle is correct. A whole seprate issue on validation of digital data.
For verifying the output my practice has been to show that the encoders of the plotter have accuratly ( with in tollerance) performed. This verification is most commonely done by outputing a scale for each of the encoders direction and then verifing the accuracy of the plotted scale with a calibrated instrument.
As stated by others if the plot is measurable you can calibrate it this way using a OC.
Once verified the recalibration is usualy limited to visual for witness lines, wear and damage.
