Validate the dynamically created textboxes?

hi anyhelp how can i validate my textboxes using javascript.. i have here my code and can anyone help me how can i validate my texbox when nothing enter on it? thanks

<script type = "text/javascript">

var counter = 0;
function AddFileUpload() {
var div = document.createElement('DIV');
div.innerHTML = 'Description' + '&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;' +
'File' + '<br />' +
GetDynamicTextBox("") + '&nbsp; &nbsp;' +
'<input id="file' + counter + '" name = "file' + counter +
'" type="file" />' +
'<input id="Button' + counter + '" type="button" ' +
'value="Remove" onclick = "RemoveFileUpload(this)" />';
...