Testing for number

\documentclass{article}
\def\isnum#1{%
  \if!\ifnum9<1#1!\else_\fi
    \emph{#1}\else#1\fi}

\begin{document}

\isnum{dummy}
\isnum{123}

\end{document}

if #1 is a number we have \ifnum9<1xxx which s true and therefore empty which leads to \if!! which is also true and \emph{#1} is the output. In the other case we have (#1 mybe 0a) \ifnum9<10a which is true and leaves a. Therefore we compare \if!a which is wrong, the reason why now the else part the output is.

Here's a slightly flawed, but slightly more generic thing than you're asking for.

\documentclass{article}
\usepackage{etoolbox}
\makeatletter
\newcommand\ifnumber[1]{%
        \begingroup
        \edef\temp{#1}%
        \expandafter\ifstrempty\expandafter{\temp}
                {\endgroup\@secondoftwo}
                {\expandafter\ifnumber@i\temp\@nnil}%
}
\def\ifnumber@i#1#2\@nnil{%
        \if-#1%
                \ifstrempty{#2}
                        {\def\temp{X}}
                        {\def\temp{#2}}%
        \else
                \def\temp{#1#2}%
        \fi
        \afterassignment\ifnumhelper
        \count@0\temp\relax\@nnil
        \endgroup
}

\def\numrelax{\relax}%
\def\ifnumhelper#1\@nnil{%
        \def\temp{#1}%
        \ifx\temp\numrelax
                \aftergroup\@firstoftwo
        \else
                \aftergroup\@secondoftwo
        \fi
}
\makeatother

\newcommand\testnumber[1]{#1: \ifnumber{#1}{Number}{Not a number}\par}
\begin{document}
\def\foo{-55}
\testnumber{1234}
\testnumber{\foo}
\testnumber{-}
\testnumber{}
\testnumber{1}
\testnumber{1234abc}
\testnumber{abc1234}
\end{document}

It's slightly complicated by checking if the first token in the expansion of the argument is a -. Unfortunately, it does not work if the argument is a register. (It probably doesn't work in other cases too.)

But from the \ifnumber macro, you should easily be about to build what you want.

\newcommand\domorestuffifnumber[1]{\ifnumber{#1}{\emph{#1}}{#1}}

A LuaTeX solution:

\documentclass{article}
\begin{document}

\directlua{ dofile("myluastuff.lua") }

\newcommand\domorestuffifnumber[1]{%
  \directlua{ 
    domorestuffifnumber("#1") 
}}

\def\foo{-55}
\domorestuffifnumber{1234}
\domorestuffifnumber{\foo}
\domorestuffifnumber{-}
\domorestuffifnumber{}
\domorestuffifnumber{1}
\domorestuffifnumber{1234abc}
\domorestuffifnumber{abc1234}
\end{document}

and the myluastuff.lua file:

function domorestuffifnumber( arg )
  if tonumber(arg) then 
    tex.sprint("\\emph{" .. arg .. "}") 
  else
    tex.sprint(arg or "")
 end
end

I think the solution is quite readable.