GetAssemblyName

But, maybe you don't need to load that Assembly, anyway. If you just need info about it, you could call AssemblyName.GetAssemblyName() with the path to the file. That will temporarily load the file to get the info, and then immediately unload it. That call will not cause it to show up as an Assembly in the AppDomain. [Suzanne Cook - Unloading an Assembly

Ah, so that is how you do it!  I was going through all sorts of convoluted ways to simply find out if a DLL was managed (without loading it).  It looks like that one call will do the trick.