Meta-programming detecting anonymous unions inside structs.

[TheFlyingFiddle via Digitalmars-d-learn]

I am trying to port a serialization library I wrote in Lua some 
time ago. I've ran into a problem relating to types with 
anonymous unions inside.

Given this code:
enum Kind
{
   none = 0,
   array,
   integer,
   floating,
}
struct Foo
{
   Kind type;
   union
   {
       ulong  integer;
       double floating;
       void[] array;
   }
   int nonUnionField;
   //...
}

How can I tell that "integer", "floating" and "array" are part 
the union while "nonUnionField" is not?

Thanks in advance.