TikZ get current unit length of x-coordinate

Here is a solution using the calc library:

\documentclass[border=2pt]{standalone}

\usepackage{tikz}
\usetikzlibrary{calc}

\def\getxlengthincm#1{
  \path let \p{x}=(1,0), \n{xlencm}={scalar(veclen(\x{x},\y{x})/1cm)}
  in \pgfextra{\xdef#1{\n{xlencm}}};
}

\begin{document}
\begin{tikzpicture}[x=2cm,y=3cm]
  \getxlengthincm{\myxlength}
  \node[align=center] at (0,0) {x length:\myxlength cm};
\end{tikzpicture}
\end{document}

Here is a variant with explicit pt unit:

\def\getxlength#1{
  \path let \p{x}=(1,0), \n{xlen}={veclen(\x{x},\y{x})}
  in \pgfextra{\xdef#1{\n{xlen}}};
}

Using the printlen package and https://tex.stackexchange.com/a/15996/36296

\documentclass[border=2pt]{standalone}

\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{printlen}

\newlength{\pgfx}
\newlength{\pgfy}

\uselengthunit{cm}
\begin{document}

  \begin{tikzpicture}[x=2cm,y=3cm]
    \node at (0,0) {};
    \pgfpointxy{1}{1};
    \makeatletter
    \setlength{\pgfx}{\pgf@x}
    \setlength{\pgfy}{\pgf@y}
    \makeatother
    \node {\printlength{\pgfx}~\printlength{\pgfy}}; 
  \end{tikzpicture}
\end{document}

\documentclass[border=2pt]{standalone}

\usepackage{tikz,luatex85,siunitx}

\makeatletter
\newcommand{\NodeDist}[3][\MyDist]{%
    \pgfpointdiff{\pgfpointanchor{#2}{center}}
                 {\pgfpointanchor{#3}{center}}
    % no need to use a new dimen
    \pgf@xa=\pgf@x
    \pgf@ya=\pgf@y
    % to convert from pt to cm   
    \pgfmathparse{veclen(\pgf@xa,\pgf@ya)/28.45274}
    \global\let#1\pgfmathresult % we need a global macro    
}

\newcommand{\TZUnits}{
    \coordinate (@0) at (0,0) ;
    \coordinate (@X) at (1,0) ;
    \coordinate (@Y) at (0,1) ;
    \NodeDist[\Xunit]{@0}{@X}\edef\Xunit{\SI{\Xunit}{\cm}}
    \NodeDist[\Yunit]{@0}{@Y}\edef\Yunit{\SI{\Yunit}{\cm}}
}

\makeatother

\begin{document}

  \begin{tikzpicture}[x=2cm,y=3cm]
    \TZUnits
    \node {\Xunit~\Yunit}; 
  \end{tikzpicture}

\end{document}