That would depend on what type of "part" you are refering to, \chaptername, \sectionname, \thepage, \thesection, \thechapter, etc... See this link for details.
EDIT: In light of your comment, this thread seems to be discussing a solution (note: there is a typo, it must be \renewcommand{\part}..., i.e. a backslash must be added):
For using the parts, you can do the
following:
\let\Oldpart\part
\newcommand{\parttitle}{}
\renewcommand{part}[1]{\Oldpart{#1}\renewcommand{\parttitle}{#1}}
and then use \parttitle in the header.
Another alternative is to use the
titlesec package which provides
commands like \partmark that you can
use.