How are DNA polymerase error rates measured?
According to their website New England Biolabs use a version of the approach pioneered by Wayne Barnes, as described in:


  Kermekchiev, M.B., Tzekov, A and Barnes, W.M. (2003) Nucl. Acids Res. 31, 6139–6147


This is basically an assay for the mutation rate in a PCR-amplified lacZ (&beta;-galactosidase) gene, assayed by transforming E. coli, plating on the chromogenic &beta;-galactosidase substrate Xgal, and then scoring white colonies as mutated genes. Also according to NEB, Agilent Technologies use a similar mutational assay, but based upon the lacI (lac repressor) gene.