The pattern I have seen is the agent(s) dispatching the flight will look at space available and if they can clear you on the spot will do so. The agents at the AC Express desk are the ones that take turns dispatching the various Dash flights.

The agents elsewhere in the airport (i.e., check-in and the lounge) tend to be the ones that unless you have a clear right to the seat, send you to the gate to ask there or do the standby thing. Both amount to deferring the issue to the agent at the gate.