Its always necessary sometimes in XSLT processing that we check for things like whether a node exists to do certain operations. Checking for nodenames is the clue. This is how you do it.
<xsl:variable name="varname">
<xsl:value-of select="ParentNode/*[name()='childnodename to check for']"/>
<xsl:variable>
<xsl:if test="$varname=''"> Processing here...
<xsl:if>