The obvious here is to ensure that the lights are not faded or the lenses aren’t broken. Some people might use different bulbs which can cause electrical problems, or cause heating issues. The mechanic will be able to point these out, and check the overall condition of headlight assemblies to ensure they haven’t been replaced due to a crash.
The lights can also hint at a car’s overall electrical health too if they’re dimming or flickering oddly. To get a perfect grade, all brake lights, headlights, highbeams, turn signals and interior lights have to be working properly.