How does one address a first officer in conversation?
Call the FO 'Captain' anyway.

Both crewmembers are equally rated in the aircraft.
The FO might have more time in the aircraft than the captain or even have been a captain at a previous airline, but is sitting in the right seat because of low seniority at this carrier.
Nobody minds being referred to by a higher title.
You're a passenger, no one expects you to be able to read the bars on their uniform anyway.