How can I determine if a .NET assembly was built for x86 or x64?

You can use the CorFlags CLI tool (for instance, C:\Program Files\Microsoft SDKs\Windows\v7.0\Bin\CorFlags.exe) to determine the status of an assembly, based on its output and opening an assembly as a binary asset you should be able to determine where you need to seek to determine if the 32BIT flag is set to 1 (x86) or 0 (Any CPU or x64, depending on PE):

Option    | PE    | 32BIT
----------|-------|---------
x86       | PE32  | 1
Any CPU   | PE32  | 0
x64       | PE32+ | 0

The blog post x64 Development with .NET has some information about corflags.

Even better, you can use Module.GetPEKind to determine whether an assembly is PortableExecutableKinds value PE32Plus (64-bit), Required32Bit (32-bit and WoW), or ILOnly (any CPU) along with other attributes.

Look at System.Reflection.AssemblyName.GetAssemblyName(string assemblyFile).

You can examine assembly metadata from the returned AssemblyName instance:

Using PowerShell:

[36] C:\> [reflection.assemblyname]::GetAssemblyName("${pwd}\Microsoft.GLEE.dll") | fl

Name                  : Microsoft.GLEE
Version               : 1.0.0.0
CultureInfo           :
CodeBase              : file:///C:/projects/powershell/BuildAnalyzer/...
EscapedCodeBase       : file:///C:/projects/powershell/BuildAnalyzer/...
ProcessorArchitecture : MSIL
Flags                 : PublicKey
HashAlgorithm         : SHA1
VersionCompatibility  : SameMachine
KeyPair               :
FullName              : Microsoft.GLEE, Version=1.0.0.0, Culture=neut...

Here, ProcessorArchitecture identifies the target platform.

• Amd64: A 64-bit processor based on the x64 architecture.
• Arm: An ARM processor.
• IA64: A 64-bit Intel Itanium processor only.
• MSIL: Neutral with respect to processor and bits-per-word.
• X86: A 32-bit Intel processor, either native or in the Windows on Windows environment on a 64-bit platform (WoW64).
• None: An unknown or unspecified combination of processor and bits-per-word.

I'm using PowerShell in this example to call the method.