To debug Google datalayer, you can use the preview and debug mode in Tag Manager and select the Data Layer Tab. Here you will see the information that was pushed into the data layer. Depending on how it was pushed, you should also be able to see the data layer information using Chrome Developer Tools under the Elements tab.
You can also enable the Preview + Debug mode within GTM. You can do this through the Version management UI for your container. GTM interface for version control of containers and activating preview / debug mode for a container version. Second, see what the dataLayer object contains using your browser debugger console.
Enable preview mode
To enable preview mode, click on the arrow next to the publish button and select Preview.
Use preview mode
Once the preview mode is enabled, navigate to the site where the container is implemented in the same browser tab and you will see a debug console window at the bottom of your browser showing detailed information about your tags , including their firing status and what data is being processed.
Google: Preview and debug containers
Google Tag Manager for Single Page Applications
Codeproject: Google Tag Manager