There are only 2 ways I can conceivably think of doing this....
One using an RFID setup that designates a number (0 to 1000 lets say) the closer to 0 it is the more "green" it is, closer to 1000 the less "green" it is, but this would require a lot of guidelines and installing chips in everything...
The other would be having a baseline for every type of product, like wattage that an inefficient one would use, and have a current meter that reads the draw for devices and rates it as better or worse than the baseline
either way there is going to be a lot of classification beforehand, and just a lot of work in general