I understand that those who hold this card get 1 free checked bag when travelling on an Aeroplan reward ticket.

I am wondering how this works exactly. I will be travelling on someone else's Aeroplan reward ticket shortly. I know the fine print says that I have to be flying on my own reward ticket, but I am wondering if thats something they check, IE can I just show them my card and stand a pretty decent chance of getting a free checked bag?

Do you normally just show the card when checking your bag? Does it work on United as well as Aircanada?