Air travel: how do I know if an airline seat update was free?

I just flew through Air Canada from the United States to Toronto directly. Before boarding, I asked at the door if there are improved seats available. In response, the representative simply gave me an executive class seat without further information. There were a couple more business seats available during the flight, and it seemed that the flight was not completely full.

I later realized that this update may not have been free. The reservation was made through an agency that uses my company, using the company's credit card. I am concerned that the update is not free and has been charged to the company's credit card.

How can I know if it is a free update or not, in addition to (hopelessly unnecessarily) worrying HR?