First, what is tagging? From what I recall reading, it is an additive to the stamp itself to help with the automated sorting system? Is this correct?
Secondly, I saw in the Scott catalog that on some stamps it says tagging omitted and has a larger dollar value assigned. I have a lot of sheets and partial sheets from the 70's and 80's. Is there an easy way to identify if a stamp is tagged? A black light maybe? And what do I look for? I would hate to sell a sheet for below face value if it could be worth a lot more.
I would not recommend the use of a "black light" to detect tagging. Instead, use a short wave UV lamp for US stamps. But be forewarned, many European countries use long wave UV rather than short wave. Many UV lamps have a selector mechanism to allow viewing of either short or long wave UV.