\documentclass[letterpaper,12pt]{article}
\usepackage[nopar]{lipsum}
\newcommand{\mytext}{\lipsum[1-4]}
\RequirePackage{everyshi}
\RequirePackage{xifthen}
\newcounter{runningPage} % Not necessarily the page number!
\newcounter{tempInsertStuff}
\newcounter{tempFinishStuff}
\EveryShipout{\stepcounter{runningPage}}
\newcommand\insertStuff{%
  \setcounter{tempInsertStuff}{\value{runningPage}}%
  \setcounter{tempFinishStuff}{\value{runningPage}}%
}
\newcommand\finishStuff{%
  \strut% seems necessary?
  \setcounter{tempFinishStuff}{\value{runningPage}}%
  \ifthenelse{\equal{\value{tempInsertStuff}}{\value{tempFinishStuff}}}{}{%
    \typeout{Ahoy: `Stuff` crossed page, at or before page \thepage.^^J}%
  }%
}
\begin{document}
\mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par
\mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par
\mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par\insertStuff
\mytext \mytext \mytext \mytext \mytext \mytext \mytext \mytext \mytext \finishStuff\par \mytext\par
\mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par \mytext\par
\end{document}