This article explains how to create custom reports in google analytics, assuming you have a google analytics account that tracks your site traffic. If you don't have one, create one, and set it up on your site using the Drupal 8 google analytics module. Go to analytics.google.com. This is how your dashboard will look like :
Click on Customisation > Custom Reports, which will take you to this :
Assuming you have set up custom dimensions like shown in this article on how to set up google analytics custom dimensions in a drupal 8 site, you have to create a custom report to view the sent dimensions. Click on New Custom Report to add dimension and metrics for a new report as shown :
Save it and you can view your custom dimension. If you haven't been sending your dimensions for a while, you will have to adjust the time frame to include the current date to view your data. Otherwise, it will come up empty.
Now you should be able to view your custom dimension data in google analytics. Read the google analytics documentation for more info. If you have questions, "Click to comment" below.