The PR 1-4s were attached directly to bundles of newspapers and normally canceled with brush strokes using a thick black or blue ink that can often be actually felt on the paper. This practice stopped in 1869 so the PR 5-8s were printed after this practice stopped. It is unclear how the PR5-8s were actually used. The PR 1-4s should never have nice clean cancels or nice city cancels. These are found only on the The Counterfeits. Below is a genuine canceled PR4 on manila wrapper. The stamp was trimmed before being attached. Jim Kotanchik in his article: Use of the Newspaper and Periodical Issue of 1865 estimates there are around 20 genuine used PR1-4s in existence. This comment has led me to take a PR1-4 The PR1-4 Used Census.
Fake postmarks of the PR1-8 are numerous. New York and Boston cancels are common. These are obvious and easily tip off a fake. The 5¢ with the line though it and the 25¢ postmarks are a little more convincing since they are very similar to genuine postmarks on PR9-32s. But again, they are not the correct postmarks for a PR1-4. The 5¢ on the right is very disconcerting. The postmark is very convincing and looks like it could have been daubed on by an ink brush. However the stamp itself is fake leaving no doubt the postmark is as well.
PR 9-32, 57-89
PR 9-125 were placed in a The Post Office Page at the postoffice. Since the books were destined for destruction and should never leave official hands, many times the stamps weren't even canceled at all. If they were they were commonly marked with a pen, handstamped with a simple design (such as a star), or punch canceled. Postmarks thus exist only from books that must of been slipped out of a postoffice illegally instead of destroyed. Again, this makes genuinely postmarked stamps harder to find than real ones (which the catalogs don't reflect). The following stamps and postmarks are real.
Again, fancier postmarks or city cancels should be immediately suspect, as well as any heavy postmark or punch cancel that just happens to be exactly where the "Facsimile" overprint should be. The $60 below has a postmark that is very similar to real ones, but the stamp itself is obviously fake. The following stamps and postmarks are fake.
Pen cancels are still common on these issues but fancier cancels and even city cancels appear as well, but again, all are obviously handstamped. And again, postmarked issues are much harder to come by than unused issues (and again the catalogs don't reflect this). The city cancels are the rarest of all and constitute probably about 10% of the total used issues (I worked a long time to complete a set of 114-125 city cancels). Of interest is that I have not seen any fake stamps or postmarks of PR102-125.