Using the Network tab in developer tools: You can monitor the network requests in your browser's developer tools. If the event is firing, you should see a network request being sent to the server.
Using a debugging tool: If you're using a tool like Google Tag Manager, they often have a debug mode that you can use to see if events are firing.
Checking server logs: If you have to server logs, you can check if the event data is being received on the server side.
Remember, the collection_viewed event is available on the online store page and logs an instance where a buyer visited a product index page. Make sure you're correctly navigating to a collection page for the event to trigger.
Try out the above and let us know if you're still seeing an issue - hope this helps!