How do we know who is using the numbering exception, so we know who is ineligible for that down?
Everyone's eligibility is determined by position on numbering exception formations. Numbers essentially don't matter.
Eligibility is determined by everyone's position at the time the center puts his hands on the ball. They are locked in as ineligible no matter where they end up shifting to.
You can also become ineligible by your initial position at the snap.
So in summary, once the numbering exception is established (center puts hands on the ball) you have to memorize the players that are lined up in ineligible positions at that time. Then at the snap, you have to memorize all of the players that are lined up in ineligible positions at that time too. Put those two sets of numbers together and all of those people are ineligible receivers for the down.
*Note: all of this only applies if you have less than 5 players numbered 50-79 on your line at the snap. If you have 5 players, it's just regular football.*