How can I check if a jQuery plugin is loaded?

Generally speaking, jQuery plugins are namespaces on the jQuery scope. You could run a simple check to see if the namespace exists:

 if(jQuery().pluginName) {
     //run plugin dependent code
 }

dateJs however is not a jQuery plugin. It modifies/extends the javascript date object, and is not added as a jQuery namespace. You could check if the method you need exists, for example:

 if(Date.today) {
      //Use the dateJS today() method
 }

But you might run into problems where the API overlaps the native Date API.

If we're talking about a proper jQuery plugin (one that extends the fn namespace), then the proper way to detect the plugin would be:

if(typeof $.fn.pluginname !== 'undefined') { ... }

Or because every plugin is pretty much guaranteed to have some value that equates to true, you can use the shorter

if ($.fn.pluginname) { ... }

BTW, the $ and jQuery are interchangable, as the odd-looking wrapper around a plugin demonstrates:

(function($) {
    //
})(jQuery))

the closure

(function($) {
    //
})

is followed immediately by a call to that closure 'passing' jQuery as the parameter

(jQuery)

the $ in the closure is set equal to jQuery

To detect jQuery plugins I found more accurate to use the brackets:

if(jQuery().pluginName) {
    //run plugin dependent code
}