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 in principle be distributed as a working version. (In practice, any distributor would test new versions against “real” documents or fonts, too; while trip and trap test bits of pathways within the program, they don’t actually test for any real world problem.)
This answer last edited: 2011-05-28