Whether you're buying it new or used, I guess the usual checks would be for overall physical condition, smooth zooming, manual focusing works, AF focusing speed and smoothness, focusing accuracy and overall sharpness since this lens is known for it's sharpness.
I've heard of some 'de-centering' issues with some Sigma lenses, so you might want to check for this.
1) Research what the 'sweet spot' apetures are,
2) Take a few shots of something flat and evenly lit. Don't stand at a diagonal angle to the object.
3) Open up the files in Adobe PhotoShop, crop at 300 dpi and zoom it to 100% to check the center and all corners.
4) If you notice a distinct difference in sharpness from the left to the right side, e.g., left corner fairly sharp and right corner blur, some of the glass elements inside could be mis-aligned and needs servicing.
Other than that, there's probably nothing much else to check for other than that the aputure blades close down fully, and respond quickly. Shoot a few shots on Manual say 1/100 speed and f22 and look directly in the lens to see if you detect any sluggishness or jamming of the blades.
The above could also apply to testing any lens, with the exception of VR/IS lenses where you would also want to put the stabalisation mechanism through all its paces with different settings.
Hope this helps.
Oh, I find a wall with lines, tiles or lots of texture a great subject to do lens testing. Use flash or high ISO if you're not sure of hand-shake.
Depends on how nitpicky you are with regards to your lens.
Yes, ultimately Dream Merchant's guide is a good one to attain lens perfection, but I don't really do that I just zoom in with my LCD preview and compare there, not professional so won't have to need that much sharpness.
Generally choose a shop that will allow you the benefit of testing. If you 'practise' this at home with your present lenses, you'll find that once you get 'used to it', you can conduct an in-shop test in less than 5 minutes.
If you ask nicely, and inform them that you wish to test the lens and inspect the image on your laptop, they should have no reason to chao tar face...unless they're trying to tell you that their customer service is bad, or if they have something to hide. In which case, you have to really ask yourself if you want to have dealings with such a shop. Of course, try to choose a day/time when you know that shop is not crowded with customers lah, and make sure you're already fast/confident in testing lens so you don't gabrah gabrah at the counter and waste time and space (hint: lots of practise at home).
What I generally do is the first thing, I take out my laptop and power up with card reader and mouse already plugged in. Launch photoshop. Or better yet, walk into shop with laptop already on standby with photoshop already launched! Say hello nicely and ask if you can test XYZ lens because you already want to buy it from them if ok. If they say ok, ask staff to take out lens. I take out my cam without lens. Attach lens and shoot a flat-on section of their counter space or display (spotted and selected first thing I walk in). Since I already know the lens' sweet spot, I shoot 2 frames wide open, and two frames stopped down to sweet apeture. All with flash. Take out card, put in reader and copy. Should take almost instantly to copy just 4 large JPEG files. Open in PS, crop and inspect. If cropped images look ok, I then 'chat' a bit while doing visual inspection and test the lens physically. If images no good, or got problem, show staff and no need waste time do physical inspection. Proceed to next copy.
If you look at the sequence, and if you're already prepared, you'll realise that it probably takes less time to 'live' test a lens than it took for you to read this entire paragraph above.
Sometimes, paying a little more and having a very supportive store to help you in case you kenar a lemon is much better than saving a few hundred and going through a lot of hassel. It's all personal choices, so choose an approach that you're most comfortable with.
All the best in your search for that perfect lens in any shop.