title: How can I check if a jQuery plugin is loaded?
body: <p>Is there any way to check if a particular plugin is available?</p>
  
  <p>Imagine that you are developing a plugin that depends on another plugin being loaded.</p>
  
  <p>For example I want the jQuery Validation plugin to use the dateJS library to check if a given date is valid. What would be the best way to detect, in the jQuery Valdation plugin if the dateJS was available?</p>