Benders are the ones that are usually background checked and tabbed, since learning bending from someone cannot bend is pretty much just as good as reading a book about bending, and are much more likely to wanting to find the avatar and help them.

Most to all of Iluq's soldiers have some sort of uniform or symbol on them so they can be identified, but those who are under cover to pretend like a civilian are given little IDs to other soldiers know.