Testing for a parameter

Hello...

May I ask, how would you check for a parameter being passed into a
stylesheet so as to invoke a template?

I would like to have a method, where if a visitor comes to a page and
has not made a choice from a drop down list, the default parameter
would be "noChoice" and this template would be replaced with a piece
of text in the table cell. I think an "if test" would be the job, but
I am unfamiliar with parameter test syntax.

<td colspan="2" class="myuweWhite">
	 <xsl:call-template name="photos">
	<xsl:with-param name="recordsInGroup"
select="//atg:record[../@groupName=$group_name]" />
	 </xsl:call-template>
</td>

Any advice would be really appreciated.

AJ