Electrons have no color, so there is no way to reliably determine whether or not they are from a 'green' source at the point of use. Some sources may have characteristics that can aid identification, but the more stages of power conversion there are between the source and the load the more difficult it becomes to do so. Also the source characteristics can be faked, though that gets harder at higher power levels. Worst case, you've got absolutely no way of telling if the user has gaffer taped a solar panel to a klieg light running from the grid!