How are nutrition facts generated about meals?

I've always wondered how those "Nutrition Facts" labels on processed foods are generated. Is there written evidence to back up those claims, and does anyone question the food manufacturers that print these on billions of food labels every year?

The enforcement is left to the FDA, so there is no telling if Nutrition Facts policing goes on during the year, via spot checks or some other kind of enforcement.

Let's not even get into the facts printed on restaurant menus. And then the question of specifics come into play. Since 10 people could eat 10 of the same meals 10 different ways (removing this, adding that), it's hard to gauge the true nutrition contained in a restaurant meal.

Should anyone be worried about this? Not really -- but since information empowers, it would be nice to have it in excess and not in short supply. But then again, keeping eating citizens ignorant is the job of many food companies.