ppilot,
61.51 states:
(iii) Except for a recreational pilot, is acting as pilot in command of an aircraft on which more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is conducted.
At my company it is very clear who is the acting P.I.C. Although our FOs are allowed to fly from the left seat on empty legs and are typed in the aircraft, our flight-release, the duties and responsibilities, and even the paycheck make it very obvious who the operator has designated as the acting pilot in command. Even though I am fully qualfied to act as pic, I have clearly not been designated prior to flight as the acting captain while flying as a copilot.
IP