how can check whether an attribute exists or not?

Can anyone please tell me how to test whether an attribute exists? Cos I
want to test it in a 'when' tags
<xsl:choose>
<xsl:when test="@href=''">
....
</xsl:when>
<xsl:otherwise>
....
</xsl:otherwise>
</xsl:choose>
And in the XML some tags have the atribute href where as some don't.
and what i did above didn't work if I didn't have the attribute href="" in
the XML for the tags that doesn't have the href value.

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