Any \start...
is good enough to identify something as ConTeXt and not Plain, if you do not care about old-school AMSTeX files.
If you want complete commands, there are quite a few top-level document delimiters in ConTeXt:
The usual document delimiters
Special page environments
These create individual pages, but of course such a page may constitute an entire document
\startTEXpage
— a page that exactly fits the TeX output it contains
\startMPpage
— a page that exactly fits the MetaPost graphic it contains
Other auxiliary files
\startmodule
— modules and environments are the same, really. In general, though, environments are written ad hoc, while modules are written to be reused (by others).
\startxmlsetups
— ConTeXt can take XML as input, and a file with XML setups that tell it how to translate the XML to TeX.
Incidentally, LaTeX documents need not contain a {document}
environment. LaTeX will generate an error if any output is generated before \begin{document}
, which Context does not; but a document without output can still be valid (compile without error), of course. If you access the primitive \end
command with \makeatletter\@@end
, you can end the document without resorting to LaTeX's \end{document}
.
Note from Charles I've made this CW, since by now most of the knowledge it contains comes from the comments below.