Macromedia uses this to test whether a MovieClip seems to be a Component
if (obj.setSize == undefined)

All components derived from UIObject have "top" "bottom" left" "right" properties.
These are relative to the parent movieClip that contains that component.
You can check the help files about the UIObject component for more info

Therefore, imitate Macromedia and detect components the same way they do,
and get their size using these properties.

... Then check whether your custom components have these UIObject properties,
to ensure that hitClip() can measure them the same way.
