This I feel is especially important for those who manage consoles of multiple offices/entities.
As each organization has their different nomenclatures and tagging practices, the only ways you can effectively track down all existing tags in the environment is by going to the Devices section and double clicking the Tags column, this will present all devices tagged in descending order, or by using the Actions option and clicking on Apply Tags (Read-only users can’t assign the tags but can see them). Lastly one can fetch a CSV report from the Devices tag and find from there.
Point is, natively from the cloud console, there is no “dedicated” way.
It would ideal if Zone Operators, Admins could create tags arbitrarily (like as it is right now) but one can also check existing tags and which devices have been tagged with them, kind of like the Groups subsection in the Manage section.