Are there any readily observable differences between good 1930's cast
stone and natural stone?

-jc

Visually, I would look for evidence that the piece was molded because it was repeatable (are there a bunch of identical sills/lintels/belt course pieces), and then look for suspiciously uniform grain or particle size.  I suppose one could have petrographic analysis done to verify that there is or isn't Portland in the stuff, but that could get costly.

 

Ralph