xslt - XPath find if node exists

Question

Using a XPath query how do you find if a node (tag) exists at all?

For example if I needed to make sure a website page has the correct basic structure like /html/body and /html/head/title.

Answer 1

<xsl:if test="xpath-expression">...</xsl:if>

so for example

<xsl:if test="/html/body">body node exists</xsl:if>
<xsl:if test="not(/html/body)">body node missing</xsl:if>