Hello @adam3, welcome to the community!
It isn't possible to know which tags are using certain Custom Types. But you can get the list of all the tags in your repository.
Currently, you can get this list by hitting the nd endpoint of your repository. If you go to https:/your-repo-name.cdn.prismic.io/api/v2 you'll get a JSON object with the metadata info of your repo including, the ref, the languages, and a tags property that contains an array of your tags.
Soon, this will get replaced with a new dedicated endpoint that we're currently working on. It'll allow you to get all tags.
For now, you can continue working with the current way of getting the tags.