Some reprinted PR of China-sets have secret markings in their design and although these are minute, they are an easy way to distinguish them from originals.
Let's take a look at three of those:
C7 : 1st National Postal Conference
C11 : 15th Anniversary of death of Lu Xun
C 13 : Peaceful liberation of Tibet.
Off these, C7 and C13-reprints have a different
perforation:
C7: reprints 12.5 vs originals 14
C13: reprints 14 vs originals 12.5
These two can also be identified by way of a secret mark in the reprints:
A C7-stamp with 3 lines: An original.
In this stamp there are 4 lines, so it's a reprint!C7: Reprints have an
extra line under the
- character in the text above: the second character from the left. Reprints have 4 horizontal lines, Originals just 3 lines.


C13: Reprints have a
very small, extra Chinese marking in the bottom left corner of the vignette. That's just under the value, under the wall/ in the grass. It resembles a tiny spider.

C11-reprints can only be identified by way of a secret marking:
a small dot in the right hand bottom margin corner. This stamp has no dot in that corner, so it's an original.

Used originals of all these sets almost always have big, black, smudgy cancellations, vs. light CTO- corner cancellations in reprints. The more beaten and heavily stamped it is, the more likely it is you have found an original!
Values for these sets:C7: Originals: 30 mint/10 used, Reprints: 2 mint/1 used
C7NE (Northeast China-issue): Originals 60 mint/50 used, Reprints: 2 mint/1 used
C11: Originals: 10 mint/6 used, Reprints: 2 mint/1 used
C13: Originals: 40 mint/25 used, Reprints: 8 mint/2 used
C7, C13 and especially C7NE are much more valuable when original!