This is not rocket science nor brain surgery. I just never needed this and found it practical to note this for further reference.
Setup: I have an XML file and a doctype, each in some place on my hard drive. I want to check whether the document validates against the doctype.
xmllint --path ./path/to/dir/with/doctype path/to/my.xml --validIf I don't want the formatted output (which I usually don't), there is an option for that: --noout will suppress printing of the result tree.