Access to the parts of glue registry

if you have etex extensions enabled you can do

\newskip\myskip
\myskip = 10pt plus 4pt minus 2pt

\edef\z{\the\gluestretch\myskip}
\show\z
\edef\z{\the\glueshrink\myskip}
\show\z
\bye

which produces

> \z=macro:
->4.0pt.
l.5 \show\z

? 
> \z=macro:
->2.0pt.
l.7 \show\z

? 

You have to take care of infinite glue:

\documentclass{article}

\makeatletter
\newcommand{\extractnatural}[1]{%
  \the\dimexpr#1\relax
}
\newcommand{\extractstretch}[1]{%
  \strip@pt\dimexpr\gluestretch#1\relax
  \ifcase\gluestretchorder#1pt\or fil\or fill\or filll\fi
}
\newcommand{\extractshrink}[1]{%
  \strip@pt\dimexpr\glueshrink#1\relax
  \ifcase\glueshrinkorder#1pt\or fil\or fill\or filll\fi
}
\makeatother

\newlength{\test}

\begin{document}

\setlength{\test}{10pt plus 4pt minus 2pt}
\texttt{\the\test}: (\extractnatural{\test}) (\extractstretch{\test}) (\extractshrink{\test})

\setlength{\test}{10pt plus 4fil minus 2pt}
\texttt{\the\test}: (\extractnatural{\test}) (\extractstretch{\test}) (\extractshrink{\test})

\setlength{\test}{10pt plus 4filll minus 2fill}
\texttt{\the\test}: (\extractnatural{\test}) (\extractstretch{\test}) (\extractshrink{\test})

\edef\macro{(\extractnatural{\test}) (\extractstretch{\test}) (\extractshrink{\test})}
\texttt{\meaning\macro}

\end{document}