Bold \checkmark

If the outline of the checkmark glyph is not only filled but stroked with a line width, then the symbol becomes bolder. The following example uses this method with the help of package pdfrender:

\documentclass{article}
\usepackage{amssymb}
\usepackage{pdfrender}
\newcommand*{\boldcheckmark}{%
  \textpdfrender{
    TextRenderingMode=FillStroke,
    LineWidth=.5pt, % half of the line width is outside the normal glyph
  }{\checkmark}%
}
\begin{document}
  $\checkmark$ $\boldcheckmark$
\end{document}

The result is a smooth bold character:

Since there is no bold checkmark provided by the fonts, there is only the last ressort of Poor man's bold provided by the package amsbsy:

\documentclass{minimal}
\usepackage{amssymb}
\usepackage{amsbsy}
\errorcontextlines=10
\begin{document}
$\checkmark$ $\pmb{\checkmark}$
\end{document}