Detecting which version of the LaTeX format is in use

Recent versions of pdfTeX, and I think all versions of XeTeX and LuaTeX, provide 'version' data for the engine. Something like

\documentclass{article}
\usepackage{ifluatex,ifxetex}
\makeatletter
\def\engineversion@#1#2.{%
  \ifnum#2>9\relax
    #1.#2.%
  \else
    0.#1#2.%  
  \fi
}
\def\engineversion@@#1#2#3{#1.#2#3}
\edef\engineversion{%
  \ifxetex
    XeTeX:\the\XeTeXversion\XeTeXrevision
  \else
    \ifluatex
      LuaTeX:\expandafter\engineversion@\the\luatexversion.\luatexrevision
    \else
      pdfTeX:\expandafter\engineversion@@\the\pdftexversion.\pdftexrevision
    \fi
  \fi
}
\makeatother
\show\engineversion

will show it in a 'nice' way. What this warns you about is that telling what engine version is in use also means worrying about which engine is in use. Without more context, it's hard to be sure quite what effect is required. One possible approach is something like

\def\@ifpdftexlater#1{%
  \ifxetex
    \expandafter\@secondoftwo
  \else
    \ifluatex
      \expandafter\expandafter\expandafter\@secondoftwo
    \else
      \ifnum#1>\pdftexversion
        \expandafter\expandafter\expandafter\expandafter
          \expandafter\expandafter\expandafter\@secondoftwo
      \else
        \expandafter\expandafter\expandafter\expandafter
          \expandafter\expandafter\expandafter\@firstoftwo
      \fi
    \fi
  \fi
}
% Use cases
\@ifpdftexlater{150}{\TRUE}{\FALSE} % TRUE on my system
\@ifpdftexlater{150}{\TRUE}{\FALSE} % FALSE on my system

which tests the major pdfTeX version, and is TRUE for any major version greater or equal to the given argument, and FALSE for older versions or for the wrong engine begin used. (This needs the ifluatex and ifxetex packages.) Tests for minor revision is a bit more complex, but the same approach applies.

If you actually want the TeX version, then the banner or similar is the only way to go. Knuth did not provide a \texversion macro, and while it's possible to tell TeX2 from TeX3, that is not much use (as TeX3 was released in 1982!).

(Second answer, as it seems the question was not really about TeX version.)

The LaTeX format version is stored in the macro \fmtversion. LaTeX does not provide a direct interface to do a date test on this, but it not too hard to put one together:

\newcommand*\@iflatexlater{\@ifl@t@r\fmtversion}

This is used

\@iflatexlater{2001/09/01}{\TRUE}{\FALSE} % TRUE, I hope!
\@iflatexlater{2011/09/01}{\TRUE}{\FALSE} % FALSE

If you use pdftex in either PDF or DVI mode (i.e. pdflatex or latex with a modern LaTeX) you could adapt Herbert's answer to my question Is there a way to detect from inside a package that MiKTeX is used? to extract the TeX version instead.

It uses the primitive \pdftexbanner which prints something like:

This is pdfTeX, Version 3.1415926-2.3-1.40.12 (TeX Live 2011) kpathsea version 6.0.1

The 3.1415926 is the TeX version and can be extracted as shown in the above link. However, this wouldn't work, in general, with a very old TeX where pdftex isn't used in DVI mode.