Check for a valid file before using \includegraphics

Second attempt

This here should do the same tests than \includegraphics. That means if will give yes if the graphics/file exists and if the extension can be handled be the engine (so tiger.eps will give different results with pdflatex and latex: (The code is from the definition of \Ginclude@graphics in graphics.sty).

\documentclass[]{article}
\usepackage{graphicx}

\makeatletter
\newif\ifgraphicexist

\catcode`\*=11
\newcommand\imagetest[1]{%
 \begingroup
 \global\graphicexisttrue
   \let\input@path\Ginput@path
  \filename@parse{#1}%
  \ifx\filename@ext\relax
    \@for\Gin@temp:=\Gin@extensions\do{%
      \ifx\Gin@ext\relax
        \Gin@getbase\Gin@temp
      \fi}%
  \else
    \Gin@getbase{\Gin@sepdefault\filename@ext}%
    \ifx\Gin@ext\relax
       \global\graphicexistfalse
       \def\Gin@base{\filename@area\filename@base}%
       \edef\Gin@ext{\Gin@sepdefault\filename@ext}%
    \fi
  \fi
  \ifx\Gin@ext\relax
         \global\graphicexistfalse
    \else 
       \@ifundefined{Gin@rule@\Gin@ext}%
         {\global\graphicexistfalse}%
         {}%
    \fi  
  \ifx\Gin@ext\relax 
   \gdef\imageextension{unknown}%
  \else
   \xdef\imageextension{\Gin@ext}%
  \fi 
 \endgroup 
 \ifgraphicexist
  \expandafter \@firstoftwo
 \else
  \expandafter \@secondoftwo
 \fi 
 } 
\catcode`\*=12
\makeatother 

\begin{document}

\imagetest{tiger}{Yes, \imageextension}{No, \imageextension}

\imagetest{tiger.eps}{Yes, \imageextension}{No, \imageextension}

\imagetest{fail}{Yes, \imageextension}{No, \imageextension}

\imagetest{failxxx.eps}{Yes, \imageextension}{No, \imageextension}

\imagetest{bib.bib}{Yes, \imageextension}{No, \imageextension}
\end{document}

it needs the full name of the file. You can scan the logfile for Imagefile to get all missing images files.

\documentclass{book}
\usepackage{graphicx}
\newcommand\IG[2][]{\IfFileExists{#2}
  {\includegraphics[#1]{#2}}
  {\fbox{File #2 doesn't exist}%
   \message{Imagefile #2 doesn't exist^^J}}}% write a message into the log file

\begin{document}    
\IG{foo}

\IG[width=3cm]{tiger.pdf}

\end{document} 

\documentclass[preview,border=12pt,varwidth]{standalone}
\usepackage{graphicx}

\newwrite\writer
\immediate\openout\writer=missing.txt\relax

\newcommand\IG[2][]{\IfFileExists{#2}
  {\includegraphics[#1]{#2}}
  {\immediate\write\writer{\unexpanded{#2\newline}}}}


\begin{document}    
\IG{foo.ext}

\IG[width=3cm]{example-image-a.pdf}

\IG{goo.eps}

\IG{hoo.png}


\section*{Missing Graphics}
\immediate\closeout\writer
\input{missing.txt}

\end{document}