If it has been shellacked or reglossed, on the rough edge you'll probably see gloss over lapping the fibers sticking out. If there's enough it will make little webs between the sticking out fiber and the rest of the cardboard.
This is from a different hobby, but is about a very similar issue. The examples shown are somewhat extreme, but illustrate what to look for about as well as possible.
https://ezstamp.com/how-to-easily-te...een-re-gummed/