How do I put a question mark above \leq?

You can use stackrel:

\begin{equation}
  2 \stackrel{?}{\le} 3
\end{equation}
\end{document}

Or, if you use the amsmath package, you can use overset as follows:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
  2 \overset{?}{\le} 3
\end{equation}
\end{document}

\stackrel{\text{\tiny ?}}{=}

Use the accents package. You can do more fun stuff with TeX primitives, but here's the easy and most flexible way:

\documentclass{article}
\usepackage{accents}
\newcommand{\qleq}{\accentset{?}{\leq}}
\begin{document}
Test: $a \qleq b$.
\end{document}