Know if object is stage or movieclip

I’m trying to make a function for centering movieclips to other movieclips, or centering the movieclips to the stage. My problem is movieclips use _width & _height and the stage uses width & height without the underscore.

I’m passing the 2 objects as parameters to the function “center” (reference for centering, and the object I want to center), but I don’t know how to make the function know if I’m passing a movieclip or the stage.

I tried typeof which returns [object object]…

any ideas?

TIA!