The following was provided by Lawrence Elliot:
I can't say that I am too familiar with Plug-Ins. Anyway I have done some reading and here is what I have come up with.
Netscape developed the plug-In technology, and Microsoft developed the ActiveX technology. The two technologies are different in that an ActiveX control is a component you can use within an application, including a web application. On the other hand, a plug-in is specific to a browser (Netscape). You must install the plug-in and restart the browser before the plug-in will become effective.
The browser (Internet Explorer) will automatically download and install the ActiveX control if the control does not already exist on the users system. In fact, depending on the browser's settings, the browser will ask the user to confirm whether the browser should download and install the control.
This being the case, it would seem that one doesn't need to detect ActiveX in Internet Explorer.
I understand that you can use ActiveX components in Netscape by using a plug-in "ScriptActive" from NCompass labs.