You should be able to see and feel the thickness of the porcelain shelving at the edges of colors on a porcelain sign. The color edges of a painted sign are not very thick.
One tricky issue with this style DuPont sign is that it was also made in a painted version on metal as heavy as a porcelain sign, unembossed. Look to see if the outer edge is white with a black stripe and the field of the sign yellow. That should indicate a painted sign. If the outer edge is yellow, then a black stripe, then a white stripe and the field yellow, that should be a porcelain sign. At least that is the way the signs in my collection are.
Condition is a huge factor in value as a near perfect sign can be worth a great deal more than one that has wear and tear.
Post photos of both sides of your sign. Follow the instructions on this link:https://www.oldgas.com/forum/ubbthreads.php?ubb=showflat&Number=703369