Originally posted by volosong
Can you point me to how I'd check for decentering?
Yep ... here's one method that doesn't involve an optical testing bench ...
At a relatively short distance, tape a newspaper to a wall and take shots from a tripod at f8 when you're absolutely perpendicular to it. Turn the camera 180 degrees (90 degrees if you've a ball head may be very easy) and take more shots.
Do the same to a brick wall at a greater distance - the bigger, the better, but you still want to fill the frame. Again, you must be absolutely 'square-on'.
Now pixel peep the shots - look for a softer side.
If the soft area doesn't rotate to follow your camera movement, it's probably your error.
If it does, send it back, get another copy - a retailer will know the drill, keep the photos as evidence if required.
The shop didn't bother to test it before selling, they won't contradict you.