How would you test if something exists?

If you have:

    <FORM>
        <FIELDS>
            <HEADER>Subject Photo Page</HEADER>
            <TITLE1>This is Title1</TITLE1>
            <TITLE3>This is Title3</TITLE3>
        </FIELDS>
        <FORMPHOTOS>
            <PHOTO NUM="1">
                <FILENAME>09May001.jpg</FILENAME>
                <FILETYPE>JPEG</FILETYPE>
            </PHOTO>
            <PHOTO NUM="2">
                <FILENAME>09005.jpg</FILENAME>
                <FILETYPE>JPEG</FILETYPE>
            </PHOTO>
            <PHOTO NUM="3">
                <FILENAME>09002.jpg</FILENAME>
                <FILETYPE>JPEG</FILETYPE>
            </PHOTO>
        </FORMPHOTOS>
    </FORM>

And I want to test if a name exists:

Something like:

 <xsl:template match="FORMPHOTOS/PHOTO">
  <xsl:choose>
   <xsl:when test=". != ''">
    <attachment>
     <source>
        <xsl:choose>

         <xsl:when test="name() = concat('TITLE',current()/@NUM)   exists"> 
<--------

                    <xsl:value-of select="../../FIELDS/*[name() = 
concat('TITLE',current()/@NUM)]"/>
         </xsl:when>
         <xsl:otherwise>
                   <xsl:value-of select="../../FIELDS/HEADER"/>
         </xsl:otherwise>
        </xsl:choose>
     </source>
    </attachment>
   </xsl:when>
  </xsl:choose>
 </xsl:template>

What I want to happen here is that for the nodes that have NUM = 1 or 3 to 
use TITLE1 and TITLE3 (since they exist), otherwise use HEADER.

Thanks,

Tom