\documentclass{article}
\usepackage{amsmath}
\usepackage{trace}
\usepackage[active,displaymath,textmath,graphics,showlabels]{preview}
\begin{document}
\traceon % preview's trace option is turned off before the address@hidden@end hooks are run
\begin{equation}
  \label{eq:bar}
  x^2 = 4.
\end{equation}
\begin{multline}
  \label{eq:baz}
  x^2 \\ + y^2 = 4.
\end{multline}
\begin{align}
  \label{eq:foo1}
  x + y &= z \\
  \label{eq:foo2}
  z + w &= x
\end{align}
We now refer to the preceding equation \eqref{eq:foo2}:
\end{document}