- Open the console of your browser (see below)
 - Enter the listener code (see below)
 - Test you application and see whether console reports log "event triggered" when you would expect it
 
How to open the console?
- In Chrome, press CTRL + SHIFT + J to open the “console” tab of the Developer Tools.
 - In Safari, enter the Preferences menu. In the Advanced part, check "Show Develop menu in menu bar". Then go back to your page, and press CTRL + ALT + I to open the Web Inspector. Then click on "show console".
 - In Firefox, press CTRL + SHIFT + K (COMMAND + SHIFT + K on Mac) to open the Web Console.
 - In Opera, press CTRL + SHIFT I to open Dragonfly. Then click on "console".
 
Listener code:
window.addEventListener('custom_event_name', function (e) { console.log("event triggered"); }, false);
The example above listens for events on element "document". Please update the name of your relevant element.
The example above listens for event with name "custom_event_name". Please update the name of the custom event you test.
You can test whether your listener code works properly by the following test script:
const event = new Event('custom_event_name');
window.dispatchEvent(event);
If it works, the browser console should contain the message "event triggered".