The Asia Miles website is a bit cumbersome, but once you learn how to work it, it is a gem. After you enroll, even with a zero miles balance, you will be able to check availability for one date at a time between HKG and JNB and for a week at a time (by using the flexible date function) between the SFO, YVR and LAX and HKG.
After you register, and login, go to:
https://www.asiamiles.com/am/en/redeem/flights/seats
Click on "Cathay Pacific" to bring up their search function. It will not tell you exactly how many mileage seats are left. Rather, you have to ask for two seats and see if it yields a flight with two seats. Similarly, you can ask for four seats and see if it produces a flight. By doing a couple of searches, you can easily determine how many seats are left.
Good Luck!