Ifxetex is a project mainly written in ..., it's free.
LaTeX package to determine if XeTeX is in use
THE IFXETEX PACKAGE v0.6
This very simple package, for both Plain TeX and LaTeX, defines the ifxetex boolean for testing whether XeTeX is being used for typesetting.
Also provides the RequireXeTeX command which throws an error if XeTeX is not the engine in use.
To extract the package from the source file "ifxetex.tex",
execute latex ifxetex.ins. Executing xelatex ifxetex.tex
compiles the documentation and generates this README.
Will Robertson Copyright 2006-2010