Validation:
A parse through the W3C validator for HTML-strict will alert you whenever an ending tag happens to be missing or an element happens (through improper nesting) to be misplaced ~ just as the parse does for XHTML and XML.
Note about HTML-transitional
The parse for HTML-transitional will not always report those errors, however.
HTML-transitional is lenient. (Look carefully at its DTD: it says "dtd-loose".) An HTML-transitional document may validate although some closing tags are missing or some elements are misplaced.
So, if you are using HTML, instead of XHTML or XML, be sure to check by validating your documents for HTML-strict.