Just read two good articles on this topic:

http://*.com/questions/480696/how-to-find-if-a-native-dll-file-is-compiled-as-x64-or-x86/

http://*.com/questions/1001404/check-if-unmanaged-dll-is-32-bit-or-64-bit

To recap:

For native dlls, we can use:

dumpbin /headers cv210.dll

For .NET dlls, we can use:

corflags "C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Data.dll"

You're looking at PE and 32BIT specifically.

• Any CPU:

  PE: PE32
  32BIT: 0

• x86:

  PE: PE32
  32BIT: 1

• x64:

  PE: PE32+
  32BIT: 0