When I fly from yyz the MLL agent checks my boarding card and touches a computer monitor.
If you look at the monitor it show all of the *A cards and a few others such as D.C. This is how the billing is controlled. In smaller airports they record the guests name and type of card in a register. When passing through
YEG the other day a sign was on the counter
saying "No agent on duty please register" and the register was open on the desk with a pen.

Biggles.