TeX

pdfLaTeX

  \documentclass{article}
  \usepackage{iftex}
  \newcommand\test[1]{\par\expandafter#1\string#1: true\fi}
  \begin{document}
  \ttfamily
  \test\ifpdftex
  \test\ifluatex
  \test\ifxetex
  \test\ifptex
  \test\ifuptex
  \end{document}

LuaLaTeX

%!TEX LuaLaTeX
  \documentclass{article}
  \usepackage{iftex}
  \newcommand\test[1]{\par\expandafter#1\string#1: true\fi}
  \begin{document}
  \ttfamily
  \test\ifpdftex
  \test\ifluatex
  \test\ifxetex
  \test\ifptex
  \test\ifuptex
  \end{document}

XeLaTeX

%!TEX XeLateX
  \documentclass{article}
  \usepackage{iftex}
  \newcommand\test[1]{\par\expandafter#1\string#1: true\fi}
  \begin{document}
  \ttfamily
  \test\ifpdftex
  \test\ifluatex
  \test\ifxetex
  \test\ifptex
  \test\ifuptex
  \end{document}