Debug mode can be enabled to see GA processes in firebug console log:
Then, in firebug you will see an GA event log & error messages:
This script can now be triggered automatically in preview and debug mode:
Tip: Your container version number is embedded in the source of gtm.js – this is helpful when you’re not sure which version you are testing:
Tip: You can also output the container number into a customDimension using this new macro.
Also try this chrome plugin called dataSlayer for validating the dataLayer:
Tip: There is a useful blog post about other methods of GTM debugging & troubleshooting here.