Ask at check in
I used to fly regularly to GVA from NZ, I flew NZ to LHR and then was interlined on BA to GVA. The luggage was always transferred. It used to be interlined on Swissair as well when it existed.
NZ interlines with whatever airlines it sells joint tickets with. After all, you can interline from a regional airport in NZ onto international flights through Auckland on QF and CX in my experience.