Among obvious signs that your headlights aren't properly aimed are oncoming drivers flashing their lights at you because your lights are blinding them, or the road ahead is brightly illuminated for only 20 feet or so, meaning the headlights are aimed too low.
Suspension problems or a heavy cargo load can change your vehicle's ride height and shift one or both headlights subtly. A collision or hitting a road hazard also can move a light assembly and misalign your lights.
One way to tell if headlights are correctly aimed is to park the vehicle on a level surface and shine the headlights on a garage door or wall 25 feet ahead (some vehicles may require a different distance). The top of the low beam shining on the wall should be at or slightly below the height of the center of the headlight lens for most vehicles. You should expect the light pattern to be higher on the right side (passenger side) to illuminate road signs and lower on the driver's side to prevent blinding oncoming drivers. This should give you a good idea of whether the lights on both sides are aimed correctly.
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