They can't either. They just know that they put out X, and you used Y, so they charge you, because you contracted with them. And as long as the total of all the Ys add up to less than or equal to X, they're OK.
Another interesting fact: clean energy is only happening because of government sponsoring. They're "wasting" money by helping people set up wind turbines and other renewables and while doing so sponsoring technology development in these areas. The first wind turbines that went up cost more energy to produce than they gave during their lifetime, now they give a net surplus. Left to the free market we wouldn't have as much wind power as we do today, reducing the CO2 emissions for everybody.
1, they're doing nothing a sufficiently motivated rich individual or private charity couldn't do, and arguably more efficiently.
2, we would have wind turbines, and other clean sources of energy, when they became cost effective (ie: oil prices raised to the point that turbines, even with their low initial efficiency become worthwhile)