For Mobil you might start with the "Unmarked Reproduction and Fantasy Sign" part of this forum. I listed a few different Mobil signs in that section. If you click on the photo it will take you to another page with a description of what makes the sign a repro. and what to look for. Your question a hard one to answer in a couple sentences as there are lots of variables with Mobil pump plates!
Also use the search box on here, there have been plenty of discussions on Mobil plates in particular over the years and some hints on what to look for on an original.