To check validity you pass the document through a validating parser which should report any errors it finds. For example,
$ xmllint --valid --noout invalidhotcop.xml
invalidhotcop.xml:10: element SONG: validity error : Element
SONG content does not follow the DTD, expecting
(TITLE , COMPOSER+ , PRODUCER* , PUBLISHER* , LENGTH? ,
YEAR? , ARTIST+), got (TITLE PRODUCER PUBLISHER LENGTH
YEAR ARTIST )
</SONG>
^
A valid document:
$ xmllint --valid --noout validhotcop.xml $