How to put a big dot (and circle) over a variable?

\documentclass{article}
\usepackage{accents}
\begin{document}
$\accentset{\bullet}{b}$
\end{document}

You can do the same with the amsmath package command \overset

\usepackage{amsmath} 

$\overset{\bullet}{W}$

see also the thread How to make a larger “\dot” to change the size of the dot.

\documentclass{article}
\usepackage{stackengine}
\renewcommand\useanchorwidth{T}
\newcommand\obullet[1]{\ensurestackMath{\stackon[1pt]{#1}{\mkern2mu\bullet}}}
\newcommand\ocirc[1]{\ensurestackMath{\stackon[1pt]{#1}{\mkern2mu\circ}}}
\begin{document}
$\ocirc{W}\obullet{x}$
\end{document}

If you need it to preserve the math style...

\documentclass{article}
\usepackage{stackengine,scalerel}
\renewcommand\useanchorwidth{T}
\newcommand\obullet[1]{\ThisStyle{\ensurestackMath{%
  \stackon[1pt]{\SavedStyle#1}{\SavedStyle\kern.6\LMpt\bullet}}}}
\newcommand\ocirc[1]{\ThisStyle{\ensurestackMath{%
  \stackon[1pt]{\SavedStyle#1}{\SavedStyle\kern.6\LMpt\circ}}}}
\begin{document}
$\ocirc{W}\obullet{x};
\scriptstyle\ocirc{W}\obullet{x};
\scriptscriptstyle\ocirc{W}\obullet{x}
$
\end{document}