If it's a brand-new lens, don't bother. Just buy it. If you're not happy with it after AFMA adjustments, return it.
If it's a used lens, find the proverbial brick wall -- almost all storefronts have one somewhere, maybe the parking lot or the receiving dock or whatever. Take a picture using the best technique you can and make sure there's nothing obviously worng. You're looking for problems, not for perfection. While you're at it, try autofocusing on the cars as they're zooming past. If the lens basically tracks and everything is in the ballpark, then go for it.
If your analysis is any more rigorous than that, then your testing needs to be equally rigorous. You probably shouldn't be buying used in the first place, and you should instead be buying a half-dozen lenses and returning all but one -- and the company you're buying from should know up front that that's what you're doing so they can charge accordingly. You should also have a damned good test rig, not just some dollar bills taped to the wall.
But, mostly, you shouldn't worry. Just buy from somewhere with a good return policy, just as you'd buy anything else, and only return things that are obviously problematic.
Do you try out a half-dozen coffeemakers before buying just the right one? Washing machines? Telephones? Camera bodies? Flashes? When was the last time you measured a flash in store not only with a calibrated flashmeter but with a spectrophotometer to make sure it'll expose properly and not give an unexpected color cast? A flash with voltage problems will screw up your photos at least as much as a lens with alignment problems.
Lenses aren't so fundamentally different from anything else you buy that you should treat them any differently.
Cheers,
b&
