Hi Zamartz, the Shopify pixel helper will tell you which events were subscribed to properly by your GTM custom pixel and let you know about runtime errors.
To understand what GTM is doing after receiving the event from Shopify, there are a couple options. You can use the tag assistant to tell you which Google tag are loaded by GTM, there are chrome extensions for the specific pixel you are tracking (Facebook, TikTok, etc) or you can look at specifc network calls in the dev console. In the update, the link to the Shopify pixel helper pages has more detailed instructions on different ways to test your custom pixel.