The quickest way to check if a package is installed is to search for it with kpsewhich {package-name}.sty. So, to check for etoolbox, use

$ kpsewhich etoolbox.sty
/usr/local/texlive/2014/texmf-dist/tex/latex/etoolbox/etoolbox.sty

If it finds the package, it will output the path (just like normal which). If it doesn't find the package, it will output nothing and have a non-zero exit code.

Perhaps the most direct way to answer the question though would be to try and use the package:

\documentclass{article}
\usepackage{etoolbox}
\begin{document}
\end{document}

The above will fail if etoolbox cannot be found.

If you installed TeXLive via the Arch repositories, you can look up the installed packages in the files located in:

/var/lib/texmf/arch/installedpkgs/*.pkgs

On Manjaro there might be similar files or the same in a different (or same?) path (I don't know, because the Manjaro-Wiki includes no article about its TeXLive).