how to test a marker in an xsl:if (via a variable?)

Hey all,
I would like to make something show up in my xsl if the marker is set to a
certain value...
So I set a marker in a template and then another template is called... In
that 2nd template, i have a line like so:

<xsl:variable name="var_birthday"><fo:retrieve-marker
retrieve-class-name="birthday"/></xsl:variable>
<xsl:if test="$var_birthday != ''">
....some output
</xsl:if>

but above doesnt work as I'd expect... Can I do this without passing
xsl:param's into the template and just use the marker somehow in the if
statement?

My best idea was to  place it like above in a variable, but doesnt work like
I expected.

Thanks,
Ari