Tamron 17-50: How to test if its a dud??

Finally took the plunge & bought a Tamron 17-50 today. I must say its not quite as sharp as expected & there is a noticable band of softness across the top of the image, particularly in the extremes. This is less noticable along the bottom.

It could be my poor testing technique tho...

I have 14 days to exchange. Any tips of how to confirm whether its a dud & worth exchanging??