Edit in JSFiddle

var log = $('#log');
var log2 = $('#log2');

$('li').click(function(){
    log.text('index of collection: ' + $(this).index());
    log2.text('index of matched collection: ' + $(this).index('li.other'));
});

<ul id="foo">
    <li class="other">Beef Jerky</li>
    <li>Smoked Salmon</li>
    <li class="other">Rocky Mountain Oysters</li>
    <li class="other">Beef Jerky</li>
    <li>Smoked Salmon</li>
    <li class="other">Rocky Mountain Oysters</li>
</ul>
<h1 id="log">index of collection: waiting ...</h1>
<h1 id="log2">index of matched collection: waiting ...</h1>