How to check for absence of a node?

A. I don't find the node "format/tr[2]/td[position()>1]", or
B. The value of format/tr[2]/td[position()>1] is "NA"

-- XML input ---
     <format>
	<tr>
	  <th>th1</th>
	  <th>th2</th>
	  <th>th3</th>
	  <th>th4</th>
	  <th>th5</th>
	  <th>th6</th>
	  <th>th7</th>
	  <th>th8</th>
	</tr>
	<tr>
	  <td>timestamp1</td>
	  <td>NA</td>
	  <td>NA</td>
	  <td>NA</td>
	  <td>NA</td>
	  <td>NA</td>
	  <td>NA</td>
	  <td>NA</td>
	</tr>
      </format>
---------------
-- XSL code ---
<xsl:template match="/">

<!-- if error tag is present, process that -->
<xsl:apply-templates select="//error"/>

<!-- else if
A. I don't find the node "format/tr[2]/td[position()>1]", or
B. The value of all td's in format/tr[2]/td[position()>1] is "NA"
call error-template -->
Don't know how to code the above conditions A and B.

<!-- else,  the input XML looks good, start processing it -->
<xsl:apply-templates select="format"/>

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