Whenever you add a tag to an entry, you can also see which tags have already been used. You can also see all the tags used in projects you have access to by clicking on the tag section of an entry or by looking at the tags available in the filter for the Notebook view.
Open any entry created by you.
To view the list of tags:
Either double click the section of tags on the entry heading
Or click on the pencil icon to the top right corner of this section
At the lower section of the dropdown, you can see a section titled All tags.