After you have passed a car on the left, you should return to the right lane when you:

• See the front bumper of the other car in your rear view mirror
  Explanation:

  "Before you return to the right lane after passing a car on the left, look at your interior rear-view mirror and make sure you can see the front bumper of the vehicle you passed."
• Have put your turn signals off
• Have turned your headlights on
• See the rear of the other car in your rear view mirror