Just read two good articles on this topic:

 

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