Go to www.americanexpress.com, and then go to their travel site. You can do dummy bookings and choose CX flights, and the site will show you the class before you actually book a ticket. For the most part, the fares do match (see example below) when I try ex-HKG bookings. For itineraries ex-TPE, which I also frequently book, you might need to fiddle around to get the exact flight matches. Sometimes, Amex prices out higher, and sometimes lower. Maybe it is due to different fare classes being chosen by the Amex site vs. Cathay; the differences are too big to explain away by foreign currency exchange rates.

This is not an exact science, but is a good guide for those who want to reconfirm the fare classes.