Is there a macro which determines whether or not a command will produce printable output?

Question

In LaTeX, 'active' declarations (commands without argument) like \Gamma produce printable characters to the output stream while 'passive' ones like \itshape do not. Is there a macro (or how to write one) which, given a LaTeX declaration as argument, will resolve whether the declaration is active or passive?

I need to be able to resolve this issue in order to correctly format the rendering of an input LaTeX argument with embedded declarations.

Answer 1

This is not an answer to the question as currently stated. Given some comments of darrelld, I believe that the goal is to define a macro, \TitleCase, which puts its argument in, well, title case. This can be done with varying degrees of robustness, but one way is to read tokens one at a time and convert letters using \uppercase and \lowercase.

Here, \TitleCase@case is at all times either \uppercase or \lowercase: it is set to \uppercase at the very start of the \TitleCase command, and after each space. It is set to \lowercase by any letter.

The result is stored one piece at a time into \TitleCase@tl, and used at the end.

\makeatletter
\newcommand*{\TitleCase@tl}{}
\newcommand*{\TitleCase@case}{}
\newcommand*{\TitleCase@marker}{\TitleCase@marker}
\DeclareRobustCommand{\TitleCase}[1]
  {%
    \let\TitleCase@tl\empty
    \let\TitleCase@case\uppercase
    \TitleCase@#1\TitleCase@marker
    \TitleCase@tl
  }
\newcommand*{\TitleCase@}{\futurelet\@let@token\TitleCase@@}
\newcommand*{\TitleCase@@}
  {%
    \ifcase 0%
        \ifcat\noexpand\@let@token\bgroup 1\fi
        \ifx\@let@token\@sptoken 2\fi
        \ifx\@let@token\TitleCase@marker 3\fi
      \relax
        \expandafter\TitleCase@normal
    \or \expandafter\TitleCase@group
    \or \expandafter\TitleCase@space
    \or \expandafter\@gobble
    \fi
  }
\newcommand{\TitleCase@normal}[1]
  {%
    \TitleCase@case
      {%
        \expandafter\def\expandafter\TitleCase@tl\expandafter
          {\TitleCase@tl #1}%
      }%
    \ifcat A\noexpand\@let@token
      \let\TitleCase@case\lowercase
    \fi
    \TitleCase@
  }%
\newcommand{\TitleCase@group}[1]
  {%
    \begingroup
      \let\TitleCase@tl\empty
      \TitleCase@ #1\TitleCase@marker
      \expandafter
    \endgroup
    \expandafter\expandafter\expandafter\def
    \expandafter\expandafter\expandafter\TitleCase@tl
    \expandafter\expandafter\expandafter{%
      \expandafter\TitleCase@tl\expandafter{\TitleCase@tl}}%
    \TitleCase@
  }
\newcommand{\TitleCase@space}
  {\afterassignment\TitleCase@space@\let\@let@token= }
\newcommand{\TitleCase@space@}
  {%
    \let\TitleCase@case\uppercase
    \expandafter\expandafter\expandafter\def
    \expandafter\expandafter\expandafter\TitleCase@tl
    \expandafter\expandafter\expandafter{\expandafter\TitleCase@tl\space}%
    \TitleCase@
  }
\makeatother
\documentclass{article}
\begin{document}
  \section{\TitleCase{helLo, \itshape wo\textup{Rl}D!}}
\end{document}