\documentclass{article}

\usepackage{pgfplots}

\usepackage{pgfplots.assert}%

\begin{document}

\makeatletter

\pgfplotsutilifstartswith{Pattern}{Pattern enthaelt es}{%
	% Ah - the boolean is ok so far. Check \pgfplotsretval:
	\pgfplotsassertequalstok{ enthaelt es}{\pgfplotsretval}{Returns wrong suffix.}%
}{%
	\pgfplotsassertfail{Boolean was wrong}%
}%

\pgfplotsutilifstartswith{Someotherpattern}{Pattern enthaelt es}{%
	\pgfplotsassertfail{Boolean was wrong}%
}{%
	% Ah - the boolean is ok so far.
}%

\pgfplotsutilifstartswith{Someotherpattern}{}{%
	\pgfplotsassertfail{Boolean was wrong}%
}{%
	% Ah - the boolean is ok so far.
}%
OK.

\end{document}

