In practice, you usually can’t objectively prove a single lens is slightly below spec without professional lab equipment or a calibrated optical bench. Manufacturer MTF charts also aren’t very useful for judging whether your individual copy is acceptable.

What you can do is test for obvious problems. A truly bad copy is usually clearly bad in real use: softness far beyond expectations, uneven sharpness across the frame, focus inconsistency, or other behavior that stands out as wrong. Run standard new-lens checks: controlled shots on a tripod, good light, a flat detailed subject, several apertures and focal lengths, and repeated autofocus/manual-focus tests.

If the lens behaves consistently and produces results in line with what that class of lens should deliver, it’s probably fine. If it is a real dud, it will generally be obvious rather than a subtle “this $1600 lens looks like a very good $500 lens” situation.

So the practical answer is: test carefully for major defects, but don’t expect to precisely measure “up to spec” performance at home. If your results show clear, repeatable problems, return or service it during the warranty period.