I have a few hundred stamps from the classic bank note era. Many of them are easy to identify using the usual methods (grill, secret marks, hard vs. soft paper). I can easily tell the difference between typical hard paper and soft porous paper using a back light. (The snap test is less accurate in my opinion).
This #178 (hard paper):
This #186 (soft porous paper):
The classic opaqueness and mottling is easily seen. Now take a look at the next two photos. Are these examples of "intermediate paper" that Lester Brookman discusses in his book ("United States Postage Stamps of the 19th Century," Volume II, p 193)? The yellowness and opaqueness suggest soft porous paper, but the lack of mottling suggests hard paper.