Both Google Tag Manager and Universal Analytics have debug modes with a variety of tools that you can use to troubleshoot your integration.
Preview and Debug
Tag Manager’s debug preview mode shows tags as they are fired and their associated data layers.
Tags Fired During Checkout
Google Analytics Enhanced Ecommerce makes it possible to send sales data, and information about product impressions and promotions to the data layer, along with any Google Analytics pageview or event. You can use the values in the data layer to measure impressions and analyze customer actions.
Values in Data Layer