The best way is check our spreadsheet, in the FAQ. Linked here Cathay Pacific / Cathay Dragon Fleet Route and Configuration Guide. It has all updated birds with wifi (and a lot of other info). Click on each subfleet or scroll through and see the reg #s .

There is also a thread on the front page here where posters report any 77W flights with/without WiFi. Other posters in the know also often post the updated registry of birds equipped with the antenna. Between all the different members of the group, which also includes hobbyists, we generally know which planes have the antenna, at least within a day of it being equipped. It is called something like "Flown on a 77W lately?...." with a lot of other words. It is a long thread with a lot of posts and will be in the first page, maximum second page of this forum.

Bottom lines is you cannot predict right now on 77W, but odds are getting higher and higher.