I have a 50D and a Sigma 30mm and like that combination very much. I will say that I did have to dial in +4 on the Sigma to get it sharp at f/1.4.
If you're able to physically test a few Sigmas before buying, then I would do that. I'm not sure about taking a focus test chart. If you're testing at f/1.4, it's not quite as simple as just firing off a few photos at a test chart.
There are certainly people more experienced than me here, but I would take a few photos in the shop of people, focusing on one eye, and I would also take a few photos of something on a wall. This allows you to check focus on a flat plane. Of course, you want to be shooting perpendicularly to the subject (especially at f/1.4) when shooting flat surfaces. Just do that as best you can, since it probably won't be practical to set it up on a tripod.
I would recommend bringing a laptop so you can see the full-sized photos there in the store.
And of course, check the store's return policy in case you find out later that it's not as sharp as you thought.
And last, you can send in the Sigma for calibration. That can be a hassle (depending on where you live), but many have found it worth it.
50D - Canon 60mm Macro - Sigma 30mm - Canon 70-300mm DO IS - Canon 17-85mm IS - Canon 50mm 1.8 II