Citation with some text inside square brackets

A recommendable choice would be to use natbib or better yet biblatex. Let's see with natbib:

\begin{filecontents*}{\jobname.bib}
@book{01,
 author={Caesar, Gaius Iulius},
 title={Commentarii de bello {Gallico}},
 year={703},
}
\end{filecontents*}
\documentclass{article}
\usepackage[numbers,square]{natbib}

\begin{document}

Here's a citation \cite[See:][p.~2]{01}

Another: \cite[p.~3]{01}.

Another: \cite{01}.

\bibliographystyle{plainnat}
\bibliography{\jobname}

\end{document}

The \cite command has now two optional argument. When only one is present, it's the "post-citation"; if two are present, the first is the "pre-citation" and the second one the post-citation.

Without extra packages you can still emulate this behavior:

\begin{filecontents*}{\jobname.bib}
@book{01,
 author={Caesar, Gaius Iulius},
 title={Commentarii de bello {Gallico}},
 year={703},
}
\end{filecontents*}
\documentclass{article}

\makeatletter
\let\cite\relax
\DeclareRobustCommand{\cite}{%
  \let\new@cite@pre\@gobble
  \@ifnextchar[\new@cite{\@citex[]}}
\def\new@cite[#1]{\@ifnextchar[{\new@citea{#1}}{\@citex[#1]}}
\def\new@citea#1{\def\new@cite@pre{#1}\@citex}
\def\@cite#1#2{[{\new@cite@pre\space#1\if\relax\detokenize{#2}\relax\else, #2\fi}]}
\makeatother

\begin{document}

Here's a citation \cite[See:][p.~2]{01}

Another: \cite[p.~3]{01}.

Another: \cite{01}.

Again: \cite[See:][]{01}

\bibliographystyle{plain}
\bibliography{\jobname}

\end{document}

In case you want only the "pre-citation", use

\cite[See:][]{01}

with an empty second optional argument.