Latex does not offer if/else, but Tex offer the ability to have conditional if/else statements, Here is a small snippet how to use it.
\documentclass[a4paper,11pt]{article}
\def \UserName {Devnodes}
\begin{document}
\ifx\UserName\undefined
\textcolor{red}{Unknown User}
\else
\UserName
\fi
\end{document}