if not exists:

<xsl:if test="not (/rows/row/@user)">
...
</xsl:if>

if exists:

<xsl:if test="/rows/row/@user">
...
</xsl:if>

if…then…else:

<xsl:choose>
    <xsl:when test="name[.!='']">
        <xsl:value-of select="name"/>
    </xsl:when>
    <xsl:otherwise>
        More Info
    </xsl:otherwise>
</xsl:choose>

Compare to a numeric value:

<xsl:choose>
    <xsl:when test="customerCount[.='0']">
    ...
    </xsl:when>
    <xsl:otherwise>
    ...
    </xsl:otherwise>
</xsl:choose>

xsl:if test

// Test for the presence of the attribute @var
<xsl:if test="@var">...</xsl:if> <!-- only true if the @var is present -->
 
// Test to see if the value of attribute var is not ""
<xsl:if test="string-length(@var) &gt; 0">...</xsl:if>
 
// Trim/compress the spaces in the attribute such that "     "
<xsl:if test="string-length(normalize-space(@var)) &gt; 0">...</xsl:if>

Check if the field CategoryName has any text:

<xsl:if test="CategoryName/text()">X</xsl:if>
<xsl:if test="CategoryName != ''">Y</xsl:if>