测试jqueryui是否已加载

if (jQuery.ui) {
  // UI loaded
}

if (typeof jQuery.ui != 'undefined') {
  // UI loaded
}

jQuery.ui
jQuery.ui.version

Firebug

function checkJqueryUI() {
    if (typeof jQuery.ui != 'undefined') {
        do_jqueryui();
    }
    else {
        window.setTimeout( checkJqueryUI, 50 );
    }
}
// Put all your jQuery UI stuff in this function
function do_jqueryui() {
    // Example:
    $( "#yourId" ).dialog();
}
checkJqueryUI();

<script src="jquery.js"></script>
<script src="jquery-ui.js"></script>
<script>
  $(function(){
    // did the UI load?
    console.log(jQuery.ui);
  });
</script>

if (typeof jQuery.ui == 'undefined') {
   // jQuery UI IS NOT loaded, do stuff here.
}

if (typeof jQuery.ui != 'function') {
    // jQuery UI IS NOT loaded, do stuff here.
}

if (jQuery.ui) {
    // This will throw an error in STRICT MODE if jQuery UI is not loaded, so don't use if using strict mode
    alert("jquery UI is loaded");
} else {
    alert("Not loaded");
}