<table>
<tr>
<td colspan="2" id="First">
</td>
<td colspan="2" id="Second">
</td>
</tr>
</table>

var index = 0;
$("#Second").prevAll("td").each(function() {
    index += this.colSpan;
});
console.log(index);

<table>
    <tr>
        <td>One column wide</td>
        <td colspan="2">Two columns wide</td>
        <td>One column wide</td>
        <td colspan="2" id="Last">Two columns wide</td>
    </tr>
</table>

<table>
    <tr>
        <td>One column wide</td>
        <td colspan="2">Two columns wide</td>
        <td>One column wide</td>
        <td colspan="2" id="Last">Two columns wide</td>
    </tr>
</table>