How can paper recyclers know how many times a given input paper has been recycled?
I've read that paper can be recycled "up to 6 times." I'm imagining a paper recycling plant that gets a ton of newspapers, books, cardboard, gift wrapping, and other household and industrial paper waste in their input stream.
If there is a hard limit on "6 recycling" for paper, I imagine the recycling plant would need to reject items that have been recycled too many times.
How can a recycling plant possibly know how many times a given input paper source has already been recycled to ensure the output paper stays within the recycle-no-more-than-6-times limit?