Is it safe to "renewcommand" \SS? (Which packages use \SS command?)

If you want to see, what a command is used for, you can use the \show macro:

% arara: pdflatex

\documentclass{article}

\begin{document}
%\show\cS
\show\SS
\end{document}

This will show:

\SS=macro:
->\T1-cmd \SS \T1\SS .
l.7 \show\SS

Ok, this information is not so cool, but you can see that is has something to do with the T1 encoding and you see that it is used as a macro. Googling the same macro will easily result in the definition (the German upper-case sz-letter). The command \cS as recommended in the comments results in:

\cS=undefined.
l.6 \show\cS

and thus is safe to be used.

Concerning the safety of re-defining command \SS (which is the capital of "ß") in order to shorten the command \mathcal{S}, @egreg said:

"Not really safe: your bibliographic data might contain a ß that in turn might be capitalized: the final result would be at least puzzling."

That is definitely true, so I've decided to follow the suggestion given by @PrzemysławScherwentke:

"And what about the name \cS, as calligraphic S? In standard LaTeX it is not defined."

In fact \cS command, as is shown by @LaRiFaRi (and by \show\cS), is safe:

\cS=undefined.
l.6 \show\cS

Thank you all: I change my code from \renewcommand{\SS}{\mathcal{S}} to \newcommand{\cS}{\mathcal{S}}.

Corrado.