The package, which works both for Plain TeX and for LaTeX,
defines the \ifPDFTeX, \ifXeTeX, and \ifLuaTeX conditionals for
testing which engine is being used for typesetting. The package
also provides the \RequirePDFTeX, \RequireXeTeX, and
\RequireLuaTeX commands which throw an error if pdfTeX, XeTeX
or LuaTeX (respectively) is not the engine in use.

Homepage:
https://ctan.org/pkg/iftex
