Roger Cicala (lensrentals.com) wrote up a nice piece on testing a new lens. Minimally, test for sharpness (shoot something with lots of detail from a reasonable distance, manually focus with 10x Live View to rule out AF errors), test for decentering (can be done with vignetting, although that's tough with this lens on APS-C - you get the most vignetting at 400mm f/5.6, shoot a blue sky and look for symmetry; else, shoot something flat with the lens well-aligned to the subject, check that the sharpness in all four corners is the same).