TeX (and Metafont and MetaPost) are written in a 'literate' programming language called Web which is designed to be portable across a wide range of computer systems. How, then, is a new version of TeX checked?
Of course, any sensible software implementor will have his own suite of tests to check that his software runs: those who port TeX and its friends to other platforms do indeed perform such tests.
Knuth, however, provides a 'conformance test' for both TeX
(trip) and Metafont (trap).
He characterises these as 'torture tests': they are designed not to
check the obvious things that ordinary typeset documents, or font
designs, will exercise, but rather to explore small alleyways off the
main path through the code of TeX. They are, to the casual reader,
pretty incomprehensible!
Once an implementation of TeX has passed its trip test, or an
implementation of Metafont has passed its trap test, then it may
reasonably be distributed as a working version.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=triptrap