Using mono-ceci, you can do this (the information is available module by module):

AssemblyDefinition asm = AssemblyFactory.GetAssembly("myassembly.dll");

foreach (ModuleDefinition module in asm.Modules)
{
    Console.WriteLine("Module " + module.Name);
    Console.WriteLine("IsPE64 " + module.Image.PEOptionalHeader.StandardFields.IsPE64);
}