You determine what specifications you want to test the product to and develop a test plan to test those specifications in a repeatable and methodical way. Ideally you use industry standard tests that allow for more direct comparisons, but if they aren't sufficient you develop your own.

For example, with his bending test. You would define a standard loading method (e.g. 3 point or 4 point with specific spacing), and then apply a realistic load a phone may experience plus some safety factor with a specific loading/unloading profile (say a phone is sat on and experiences 100lbf you might test to 125lbf). Once released you inspect the phone for damage, deformation, and test functionality.