What I always do is to go to the gate of the earlier flight and politely told the agent that I wanted to standby for this flight only if first class is available (I am confirmed in F for a later flight). It looks that almost all the times I made this request, I got a first class BP immediately from the agent. The only time I missed a F was to standby a CO flight EWR-FLL when I was a silver; also I was too close to the departure time that day.