Including different content based on whether xelatex or pdflatex is invoked [duplicate]

Possible Duplicate:
Can I write a document that is compilable with both latex and xetex?

I use XeLaTeX for my documents, and have a font set that I like. However, I also work with coauthors who aren't quite ready for xelatex yet, and use PDFLaTeX (or even plain LaTeX).

I'd like to construct a macro that includes certain content (fontspec and certain font packages) if I use XeLaTeX and other content if using PDFLaTeX/LaTeX. For example, something like

``````\if <*using xelatex*>
\usepackage{fontspec}
\setmainfont{<*my favorite font*>}
\else
\usepackage{mathpazo}
\fi
``````

How would I set this up?