Well I'm no expert - never found a need to subclass a window. To do it you need to intentionally make changes, overriding the original so it is not done by itself, or accidentally, as far as I know. Other than looking at code I don't know if there is a flag or property that indicates a subclassed window or control.
When I run your mGimp program which became Cfe, I count 6 windows before I even try to apply a filter.
John Z