Find out what the IP-address of your Chromecast device is. (Tip: Use the Google Home app to look this up.)
Go to chrome://inspect and add the IP-address (plus port 9222) of your Chromecast device.
Start your Chromecast session.
Go to chrome://inspect and wait until your Chromecast device appears in the list.
You should now have access to the Chrome Developer Tools of your Chromecast Receiver application. If you are familiar with debugging through Chrome's Developer Tools, you can look into issues, try out new CSS styles and monitor network traffic.