This is a question like the catchy ones from job interviews.
Do you need this on configurable product page on frontend ?
var spConfig = new Product.Config(<?php echo $this->getJsonConfig() ?>);
You need to extend this object. Either add your function in the class or extend an existing function that you'll need. http://prototypejs.org/learn/class-inheritance
In the JSON config passed to spConfig by default there is the
options key and a deeper
products key. You can use this config in your JS function. On any configurable product page copy paste the JSON object and format it with a tool for ease of understanding.
The JSON is built in
In the JS class there is:
Event.observe(element, 'change', this.configure.bind(this))
configureElement function there is the call:
reloadPrice an iteration on the dropdowns is made.
Your code should mimic this iteration to check if all available dropdowns were selected.
On success your custom code comes in.
Is it an Ajax call to fetch dynamic content ? I would not recommend it. Instead try to have the dynamic content/data of associated products available in the rendered page in a JSON object handled by you custom JS class.