As you can see under "Upgrade location", it's mostly at the gate, in the lounge or at check-in. They scan or issue your boarding pass and tell you you're upgraded. It's done by an opaque system of Skywards status, fare class, miles owned and more (based on hearsay and personal experience). Although it's not fully clear who gets upgraded first, it's very rare for non-status pax, even less on the first flight.

In any case, asking doesn't help. They only upgrade people for free if they really have to (overbooking).