To check your blind spot before changing lanes, you should:

When preparing to change lanes, checking your blind spot is essential for ensuring that there are no vehicles or obstacles hidden from your mirrors. The correct method to verify this is by looking over your shoulder in the direction you plan to move. This action allows you to see areas that are not visible in your mirrors, which can include vehicles that may be in your blind spot.

The rationale behind this approach is that mirrors can only provide a limited view of what is around your vehicle. There are specific zones, particularly immediately adjacent to your car, that mirrors might not adequately reflect. By turning your head and visually checking the lane you intend to enter, you gain a clear and reliable perspective of any vehicles that may be in that space.

Additionally, while looking into the left side or right side mirrors and the rearview mirror are good practices for overall awareness of traffic, they do not replace the need to physically check your blind spot when making a lane change. Each of those mirror checks serves its purpose, but none are as effective for blind spot verification as that direct look over your shoulder.