Welcome lynnk.
One way is looking on the QF website and finding which gate number an arrival has been allocated, and then matching that to a departure (or doing that in reverse). There are only, for instance, a limited number of A380 flights QF operates: SIN/LHR (QF1) and LAX (QF11) daily, DFW (QF7) six days a week and (over Christmas for a limited time) QF127/128 to/from HKG. In MEL it's also a small number: QF35 to SIN and QF93 to LAX daily.
Sometimes when problems occur this information can change. On occasion one doesn't find out until a flight has departed and shows up on website Flight Radar 24 (which we abbreviate to FR24).
AFF member Flyerqf is a whiz at all this and sometimes I suspect - hehe - collates information from other websites. That's perfectly OK.
lynnk, one other point is that QF's first refurbished A380, registration number VH-OQK has a differently configured seating plan to the other 11, so one can look up (for free) a flight number by date, origin and destination on the Expert Flyer website. Should it be OQK, it'll be obvious. The Seat Guru website has a map of the vatrious seating plans for each class (model) of aircraft by airline. Hope all this helps.
Any contributions you or others can make are always most welcome as I and others cannot monitor every delay or cancellation, so there are numerous occasions when I for instance miss things, or have incomplete (or sometimes incorrect) information. The more the merrier.