One thing that you can do is look at the fare codes. For AF's claim to be true (that the flown leg was much more expensive than the refunded leg) the fare code for the flown leg should also be higher than the fare code for the return leg. Your original e-ticket should include the fare codes.

Overview of all the AF the fare codes is here: https://www.airfrance.ru/RU/en/local..._structure.htm

The tables are sorted expensive-to-cheap. So Y is higher than M, and U is lower than M.