Originally Posted by PentaxPoke 
Take some pictures and compare the edge sharpness to the center. Test at different apertures and focal lengths. If one edge is sharper than the other it is decentered (Bad). If the edges are all very "soft" compared to the center, again it is bad. My first copy of the lens had the first problem, my second had the second problem. Who knows what the third one will have... Often brick walls are used for this test since you have a uniform surface that you can easily discern the problem. My lens defects were so bad, I could see the problems without a brick wall. I have 2 other threads on this, in this forum.
How far from the wall should I take my test shots? Should I zoom full-in, full-out? Apperature ISO settings?
Thanks,
Victor