Much of the technology comes from forensic reconstruction, where they do a reconstruction and then try matching it against missing people. In those cases, they do often get feedback on how well they did.
as PapaBear3625, yeah, these reconstructions are very close to the original, as they are based on the average depth of tissue over each part of the skull. If DNA is available, skin, eye, and hair color and ethnicitiy can be made much more accurate, leading to a further improvement of the likeness. When the remains are of an historical figure for whom a contemporary photo, portrait, or sculpture exists, comparisons can be made (such as happened with Robespierre, Richard III, King Tut, etc).