Abstract: Disclosed is a method of proof testing a sheet of brittle material such as a glass or glass-ceramic based material. The method comprises bending the glass sheet over at least one arcuate member to detect sheets having a strength greater than a predetermined value. The method includes imparting a bend to the sheet and producing relative motion between the sheet and the bend such that the bend traverses the sheet, and wherein tensile stress induced in a surface of the sheet by the bend corresponds to the predetermined strength value. An apparatus for performing the proof testing is also disclosed.