=== iftex package ===
{{{
\usepackage{iftex}
}}}

{{{
\ifPDFTeX
...
\else
...
\fi
}}}

{{{
\ifXeTeX
...
\else
...
\fi
}}}

{{{
\ifLuaTeX
...
\else
...
\fi
}}}

{{{
\RequirePDFTeX
\RequireXeTeX
\RequireLuaTeX
}}}

=== ifxetex, ifluatex package ===
{{{
\usepackage{ifxetex}
\ifxetex
...
\else
...
\fi
}}}

{{{
\usepackage{ifthen,ifxetex}
\ifthenelse{\boolean{xetex}}{..<true>..}{..<false>..}
}}}

{{{
\usepackage{ifluatex}
\ifluatex
...
\else
...
\fi
}}}

=== ifpdf package ===
{{{
\usepackage{ifpdf}
\ifpdf
...
\else
...
\fi
}}}