Well, until they display the fare class on the booking pages of the website the only way to find out is to call DC or res and ask them to tell you, quoting the dates, times and fares in question.
Nowadays when you check in for the outbound sector you get an itinerary receipt (printed on a boarding pass) which gives a full breakdown of fares, fares calculations, etc...but obviously this is too late to be of any use.