LOL, nop. When the plane is "full", the first member of the crew array should be the pilot, the second one should be the first cargo unit (a logic or a person), this one should be the co-pilot.
So, before checking for the copilot:
1 - Make sure the plane is full, try to add 4 logics (pilot and 3 cargo positions). These logics will use any still-free slots in the plane.
2 - Get the crew array with "crew" command and check for the second member of the array, if it is a logic, there is no co-pilot, if not, then you got the copilot.
All the above is based on the theory that once a vehicle is full, crew command will return the array ordered by driver, gunner (not present in cessna) and the ordered used positions of the vehicle cargo.