\documentclass{article}
\usepackage{ifthen}
\usepackage{xltxtra}
\setmainfont[Ligatures=TeX]{ITC Souvenir Std Light}
\makeatletter
\newcommand{\showfont}{encoding: \f@encoding{},
  family: \f@family{},
  series: \f@series{},
  shape: \f@shape{},
  size: \f@size{}
}
\newcommand{\iffont}[3]{\ifthenelse{\equal{\f@family}{#1}}{#2}{#3}}
\makeatother
\begin{document}
We are using this font: \showfont

This is normal text.

\em{This is emphasised text}

\bf{This is bold text}

We switch to Helvetica now:

\renewcommand{\familydefault}{\sfdefault} \normalfont

We are using this font: \showfont

This is normal text.

\em{This is emphasised text}

\bf{This is bold text}



\end{document}

