This is one thing most people don't think to regularly check, but can keep you from getting pulled over at night (or early morning in my case). Turn on your headlights and then head to the back of your vehicle to make sure both lights are on and that you can see your license plate in the dark.
No license plate in this picture, but you can use your imagination to see where it goes.
If either or both are out, head down to the auto parts store and get replacements. These are easy to replace and in some vehicles you don't even have to take off any paneling.