Substitutions during a timeout
 

I have been officiating for 8 years, and in my training, and in my experience, I have never heard anyone mention how to enforce the rule on not allowing subs in after a timeout, if they report with less than 15 seconds remaining.

If they report late, that's easy. But when no player reports to the table, how do you tell if a different player takes the floor? Do you look at the numbers of the players who are leaving the floor? Do you keep an eye on those five during the timeout to make sure they are the ones who come back out?

That's the scorer's job (but not all do it -- even at the V level).

If you notice some confusion as the team leaves the TO, you can ask the scorer who was in, or whether whoever comes in as part of the confusion was in / checked in.