$('li').filter(function(index) {
         return $('strong', this).length == 1;
        }).css('background-color', 'red');

for this

    > <ul>
      <li><strong>list</strong> item 1 -
        one strong tag</li>
      <li><strong>list</strong> item <strong>2</strong> -
        two <span>strong tags</span></li>
      <li>list item 3</li>
      <li>list item 4</li>
      <li>list item 5</li>
      <li>list item 6</li>
    </ul>