Here is how I test when I wanna to buy a lens in a shop.
I shoot with my kit lens. Using the same aperture and focal length, I shoot with the "better lens" that I might be buying.
On the cam's LCD, I max out the magnification and compare. "Better lens" jolly well give me better results. Otherwise, no point buying. Check centre and then, check corners and compare.
Next, move on to test at max aperture.
