When Lane Assist is on, you should see the indicator light up on your instrument cluster (green car icon with dashed lines on either side of it). I experimented with this feature a bit after getting my car, but ended up disabling it as I do mainly urban driving and found it to be rather distracting (plus I'm a very attentive driver). Though, I could see it being nice to have on longer highway drives. It does not issue a warning such as an audible alert. You will, however, feel it in the steering wheel as it course corrects. I found that at moderate speeds, it was even pretty good at steering the car through a gentle curve with my hands off the wheel, and it will definitely keep you within a well-marked lane.