You can find out if your flight has wifi by going to Flight Status on the website and type in your flight number (T-3 days prior to departure). If there is the wifi sign next to the aircraft type of your flight info then it means your plane will have wifi. Of course this is subject to last minute equipment swap but one can always check after your inbound flight has departed. For eg. if you're flying on CX252 from LHR to HKG then you just need to check after CX251 has departed the night before.
FYR I also just flew back from LHR on a 77H plane without wifi today and had the exact same question as OP. Why would they send an old plane without wifi and also has the old entertainment system (which has far less IFE choices than the new one) is beyond me considering there are so many spare frames now parking at HKIA! It was quite a scene as the plane landed at HKG and I saw all the planes on the ground!