How to Tell if a .NET Assembly Was Compiled as x86, x64 or Any CPU

If you just want to find this out on a given dll, then you can use the CorFlags tool that is part of the Windows SDK:

CorFlags.exe assembly.dll

If you want to do it using code, take a look at the GetPEKind method of the Module class:

Assembly assembly = Assembly.LoadFrom("path to dll");
PortableExecutableKinds peKind;
ImageFileMachine imageFileMachine;
assembly.ManifestModule.GetPEKind(out peKind, out imageFileMachine)

You then need to examine the peKind to check its value. See the MSDN docs for PortableExecutableKinds for more info.