When initiating the deletion of the tag, a confirmation message like "Are you sure you want to permanently delete this tag? The tag is used by # items." appears. In this way, you would know if the tag is being used by a work item or not. However, this is not very helpful when you have several tags on the list.
You can get a list of the tags by using Tag Management App. See the GitHub link
for more information.
1. Add "Custom HTML" app to your dashboard or shared page
2. Copy the HTML Code from:https://github.com/RallyTechServices/tag-management/blob/master/deploy/Ugly.txt
3. Paste the HTML code into the HTML area > Save
4. See the Custom HTML App, it should look similar to:
In the image above, tag 123 is being used and tag 1234 is not being used.