How do we know who is using the numbering exception, so we know who is ineligible for that down?
A word of advice - instead of focusing on the numbering exception, simply identify the ineligibles. On my crew, the Umpire mentally records the numbers of the snapper and two guards, I make a note of the tackles, and the wings have any others, if there are some.
Most of the time the punt formations we cover are traditional (7 on the line, 4 in the backfield), but sometimes we have seen 9 on the line with the punter and one more in the backfield...
The thing to remember is that the numbering exception was designed to allow K to replace either the snapper on 1,2,3 down, or all 5 big slow linemen on 4th down. So, essentially, the number exception only applies to 5 players. It's not necessary to identify them individually in regard to eligibility issues. The numbering exception simply keeps you from having to throw a flag for illegal numbering. What you have to pay attention to is the formation itself. That will tell you who is eligible and who is not.