To find how much you are storing in your Shared Drive go to drive.google.com. Expand the “Shared drives” item in the left sidebar. Select the Shared Drive you’re interested in, and then Choose Info (little i in a circle) in the upper right hand corner.
The Details information panel will open and you can scroll down to see the storage amount. Note: If you are using very little storage, the amount might not appear. In that case, you are under quota anyway.