Figuring out whether a .Net dll is 64bit or 32bit

 Open the Visual Studio Command Prompt and type "corflags [your assembly]". You'll get something like this:

c:\Program Files (x86)\Microsoft Visual Studio 9.0\VC>corflags "C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Data.dll"

Microsoft (R) .NET Framework CorFlags Conversion Tool. Version 3.5.21022.8 Copyright (c) Microsoft Corporation. All rights reserved.

Version : v2.0.50727
CLR Header: 2.5
PE : PE32
CorFlags : 24
ILONLY : 0
32BIT : 0
Signed : 1

You're looking at PE and 32BIT specifically.

Any CPU:

PE: PE32
32BIT: 0

x86:

PE: PE32
32BIT: 1

x64:

PE: PE32+
32BIT: 0