Take a look at the MSNMsgr.exe (version 15.4, the latest), if you take it apart and look at the RVA Data Directory section you'll see the 14th entry (reserved for the CLR runtime header's RVA) is marked as null. Meaning msnmsgr.exe contains no managed code.
All you really need to verify it is use some type of window spy program, since all WPF windows will have a class name that begins with HWndWrapper and won't have any child windows.
Part of what kills WPF for me is the lack of support for alternative visual styles, even official Microsoft styles, and instead gives you a (deliberately?) ugly Windows 2000-style look.