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:

GTM will validate JavaScript within customHTML whenever a version is created, but if you want to enable inline validation this free chrome plugin can be installed here:

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:

https://chrome.google.com/webstore/detail/dataslayer/ikbablmmjldhamhcldjjigniffkkjgpo

Tip: There is a useful blog post about other methods of GTM debugging & troubleshooting here.