How to use \verb in the environment tabular?

Here is one way. Save the verbatim stuff in a verbbox before the tabular, and then invoke it from within the tabular.

\documentclass{article}
\usepackage{array,verbatimbox}
\begin{document}
\begin{myverbbox}{\sfverb}\sffamily\end{myverbbox}
\begin{tabular}{|c|c|}\hline
\multicolumn{2}{|c|}{\sfverb}\\
\hline
\end{tabular}
\end{document}

Above, I use a named verbbox. If there is only one needed at a time, you can just use the default-named verbbox:

\documentclass{article}
\usepackage{array,verbatimbox}
\begin{document}
\begin{verbbox}\sffamily\end{verbbox}
\begin{tabular}{|c|c|}\hline
\multicolumn{2}{|c|}{\theverbbox}\\
\hline
\end{tabular}
\end{document}

You can't use \verb in the argument to another command, in this case \multicolumn.

For the specific problem, \verb is not necessary; I suggest to do

\newcommand{\cs}[1]{\texttt{\symbol{`\\}#1}}

and now

\multicolumn{2}{c|}{\cs{sffamily}}

will work.

I would use verbdef to define a macro with the verbatim content outside the tabular and then use the macro inside it.

\documentclass{article}
\usepackage{array}
\usepackage{verbdef}
\begin{document}
\verbdef\myverbtext|\sffamily|
\begin{tabular}{|c|c|}\hline
\multicolumn{2}{c|}{\myverbtext}
\end{tabular}
\end{document}

The newverbs package also provides a \collectverb{<code>}<char><verbatim><char> macro which collects the verbatim for you and feeds it to a macro. However, \collectverb{\multicolumn{2}{c|}}|\sffamily| does not work here as \collectverb is not allowed between tabular rows (! Misplaced \omit.).