If you take a coin, such as a quarter, and tap it on the sign in various places, if un-restored, it will make a metal sound. If the sign has been restored with body filler and repainted, it will sound like plastic. If the sign has been clear coated, it will also sound like plastic. Rubbing the coin sideways over the sign will scratch clear coat, but not porcelain. The eyeball test is the best method. You need to go to shows, swap meets and other venues and look at as many signs as you can to see the differences in real versus restored.