How do I determine if a .NET application is 32 or 64 bit?

To do this at runtime...

You can evaluate IntPtr.Size. If IntPtr.Size == 4 then it's 32 bit (4 x 8). If IntPtr.Size == 8 then it's 64 bit (8 x 8)

If you're trying to check whether or not a running application is running in 32-bit or 64-bit mode, open task manager and check whether or not it has an asterisk (*32) next to the name of the process.

EDIT (imported from answer by manna): As of Win8.1, the "bittyness" of a process is listed in a separate detail column labelled Platform. (Right click on any column header to expose the select columns menu.)

If you have a compiled dll and you want to check if it's compiled for 32-bit or 64-bit mode, do the following (from a related question). I would think that you want you dll to be compiled for AnyCPU.

Open 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.

AnyCpu:

PE: PE32 32BIT: 0

x86:

PE: PE32 32BIT: 1

x64:

PE: PE32+ 32BIT: 0

Well, if you're using .NET 4.0, there's System.Environment.Is64BitProcess.