To check your blind spot when changing lanes to the left, you should glance:

When changing lanes to the left, it is essential to check your blind spot, which is an area not visible through your mirrors. Glancing over your left shoulder provides you with a direct line of sight to this blind spot, allowing you to see any vehicles or obstacles that may be in that area before you move into the lane. This practice minimizes the risk of collisions and reinforces safe driving habits.

In this scenario, simply relying on side mirrors, a rearview mirror, or glancing over the right shoulder would not effectively cover the left blind spot. Side mirrors and rearview mirrors can miss the presence of vehicles directly beside you on the left, while looking over the right shoulder is not relevant when changing lanes to the left. Thus, the action of glancing over the left shoulder is critical for ensuring a safe lane change.