Follow the instructions at this link to do a focus test:
I suggest that you try several different lenses to make sure that it really is the lens and not the autofocus system that's out of whack. Also, if you can get ahold of another copy of the lens, you can check to see if it's because you just have a bad copy of the lens.
If you want to see how badly my Sigma 24-70 f2.8 EX DG Macro was out of whack, take a look at the lens tests I did here:
On "Sigma test image 4", I was focusing directly on my cat's eyes. If you look at the photo though, you can clearly see the texture of the lazyboy he's sitting on is in focus and not his eyes.
If you determine that your lens is front focusing, the only way to fix it is to send it in to Sigma to get it adjusted. Send them some prints of your photos that clearly show the problem, and they'll fix it for you.
If it's the autofocus sensor in the camera itself, you'll need to send it in to get adjusted.