[header] How to fix a low beam headlight [title] Identify a blown out bulb. [step] You can often tell when a headlight blows out while you're driving, but confirm which bulb has actually blown out by leaving your headlights on and getting out of the vehicle to check. Then get back in your vehicle and turn on your high beams. 
Some vehicles use a single bulb for both high and low beams while others do not. If the high beam on the same side is also out, it may be one bulb.