How do I know if my Sigma 17-70 Optical Stabilizer is working properly?

Hi there. I just bought this new lens for my Canon 400D digital reflex, it's the first time for me to use an optical stabilizer and I'm wondering how it works exactly. I couldn't notice any difference when the switch is turned on so far, I tried to shoot pictures at a slow speed (around 10-15) moving slightly the camera body but the result is always blurred photos. Any idea about what to do?