I looked into knob & tube wiring insulation containing asbestos a while ago. From what I can recall, it was only required in places like theaters and not residential applications. It makes sense because it would have been more expensive than the standard non-asbestos containing version. If I come across it, I just burn a small (2 - 3 inch) section to see if there is any white asbestos fiber left with the ash. So far, I haven't come across any containing asbestos, but I don't deal with it every day.