sometimes the sensor's "hotspot" is marked - with a cross or X painted in white or cut into the pavement, often in either the upper right or left hand corner. if you don't see those markings but can see the larger outline of the sensor, try placing your crank/bottom bracket at either of the upper corners. in my area placing the crank directly above the hotspot will set most of them off. the cars love it when a solo cyclist changes the light! just kidding.