From http://www.w3schools.com (Copyright Refsnes Data)

XSLT function-available() Function

Definition and Usage

The function-available() function returns a Boolean value that indicates whether the function specified is supported by the XSLT processor.

You may test the XSLT functions and the inherited XPath functions.

Syntax

boolean function-available(string)

Parameters

Parameter	Description
string	Required. Specifies the function to test

Example 1

<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:template match="/">
<html>
<body>
<xsl:choose>
<xsl:when test="function-available('sum')">
sum() is supported.
</xsl:when>
<xsl:otherwise>
sum() is not supported.
</xsl:otherwise>
</xsl:choose>
<xsl:choose>
<xsl:when test="function-available('current')">
current() is supported.
</xsl:when>
<xsl:otherwise>
current() is not supported.
</xsl:otherwise>
</xsl:choose>
</body>
</html>
</xsl:template>

</xsl:stylesheet>

View the XSL file and View the result

Complete XSLT Function Reference