Displaying All Fields in a SharePoint Data Source
So, you want to know which fields your data source is returning. You can use the following XSLT:
1: <xsl:for-each select="/dsQueryResponse/Rows/Row/@*">
2: <p>Name: <xsl:value-of select="name()" /> Value: <xsl:value-of select="." /></p>
3: </xsl:for-each>
Very handy, even if I say so myself! 
Update: this can also be useful.