This might be somewhat confusing but, I'll do my best...
I've really astute hearing, so, this is my theory. I think that it depends on the type of plane & radio type that allows you to listen to ATC on channel 9. For example, if the plane is a boeing 737, I can probably tell by the sound of the radio that they don't have the ability to listen to channel 9. On the other hand, if on an airbus A319 or a boeing 757, I can tell by the sound of the radio that they have the capability but I can't tell if it is being used. Is my theory correct? Sorry for any confusion. I'm just VERY curious.