\documentclass{article}
\usepackage{luacode}
\usepackage[T1]{fontenc}
\newif\ifunix
\begin{luacode}
function isUnix()
if package.config:sub(1,1) == '/' then
tex.print("\\unixtrue")
else
tex.print("\\unixfalse")
end
end
\end{luacode}
\directlua{isUnix()}
\begin{document}
Hello, I'm running this code in a \ifunix Unix\else Windows \fi environment.
\end{document}