Linking quotation environment with citation styles

Assuming that a) "quotation environment" actually stands for text enclosed in quotation marks b) you are willing to use the csquotes package to handle quotation marks (note that its use is recommended for biblatex), you may do the following:

• Declare a new boolean switch withintextquote (which is initially set to false);

• Patch biblatex' prenote bibmacro so that it will add the (new) bibliography string compare plus a space if the withintextquote switch is set to false;

• Patch csquotes' internal \csq@tquote@i macro so that it will set the withintextquote switch locally to true.

EDIT: The automatic compare prenote is now overridden if the user adds an explicite prenote to the citation command.

\documentclass{article}

\usepackage[ngerman]{babel}

\usepackage[style=authoryear]{biblatex}

\NewBibliographyString{compare}
\DefineBibliographyStrings{ngerman}{%
  compare = {vgl\adddot},
}

\usepackage{csquotes}
\SetCiteCommand{\autocite}

% \usepackage{etoolbox}% loaded by `biblatex` 

\newbool{withintextquote}

\makeatletter
\patchcmd{\csq@tquote@i}{\begingroup}{\begingroup\booltrue{withintextquote}}{}{}
\makeatother

\renewbibmacro*{prenote}{%
  \iffieldundef{prenote}{%
    \ifbool{withintextquote}{%
    }{%
      \bibstring{compare}\addspace
    }%
    }
    {\printfield{prenote}%
     \setunit{\prenotedelim}}}

\usepackage{filecontents}

\begin{filecontents}{\jobname.bib}
@misc{A01,
  author = {Author, A.},
  year = {2001},
  title = {Alpha},
}
\end{filecontents}

\addbibresource{\jobname.bib}

\begin{document}

This is a paraphrase \autocite{A01}

This is a paraphrase with prenote \autocite[siehe hierzu auch][]{A01}

\textcquote{A01}{This is a direct quotation}

\printbibliography

\end{document}