How to check if a fragile macro value is empty or will not create text with plain TeX conditionals?

Question

A similar question has been already asked here, but has not been answered yet for fragile commands such as eg. \@author?

What would be the right way to detect if \author has been called with a value:

\usepackage{ifthen}
\usepackage{xifthen}
\usepackage{etoolbox}

\makeatletter

\ifXXXXXX{\@author}%
{%  
    author set \par
}{%
    author not set at all \par
    or set to \{\} () \par
    or to {\textbackslash}empty
}%

\makeatother

Answer 1

\@author isn't typically a fragile command (It depends on the Author's name I suppose) However testing its value is not related to fragile/robust distinctions.

Initialise the macro, say

\let\@author\@empty

then later test if it still has that value

\ifx\@author\@empty
    still empty
 \else
     author has been set
 \fi