Unlike a car, a motorcycle does not have a straight rearview mirror. Side mirrors must be used instead. Proper adjustment of these mirrors is an important safety factor.
Sit on the motorcycle in the position normally used for riding, wearing the helmet you normally wear.
Look into the right mirror and without moving your head position, reach out and move the mirror until you can see your right hip.
Look into the right mirror and move the mirror outward until you can no longer see either the rider or the motorcycle. This mirror should now give you a partial view of what is behind you and a partial view of what is in the next lane.
Look into the left mirror. Again, without moving the rider's head position, adjust the mirror inward until you can see any part of the rider or motorcycle.
Have a friend stand five feet behind the bike. Look into the left mirror and move the left mirror outward until neither the rider, the motorcycle or the standing person are visible. This mirror should now allow you to see what is in the left lane.
Never adjust your mirrors while riding. Taking your eyes off the road and your hands off the bar are a bad idea at any speed. Wait until you have come to a stop to fine tune your adjustments.