[Closed] rotating camera and object positions

I have a camera that rotates around its local z axis – I animated the roll-parameter.
How do I calculate if an given object is left or right of the camera center at a given frame?

Is this way correct:
I calulate the angle between the loca z axis of the object and the loca y axis of the camera against a plane defined by camera.pos, object.pos and the camera local x axis?

**edit:
to make it even more interesting:
I realise that my objects are rotated around an aritrary axis, so their local z axis is of no use …

** edit

Georg