Reading `:help 'tags'` and scrolling a little bit down finds the following sentence:

> 	The *tagfiles()* function can be used to get a list of the file names
	actually used.

Accordingly, `:echo tagfiles()` will show the name(s) of found tagfile(s).

An easy way to test this is in any of Vim's help files.
It will pick up the helptags in `<whatever>/doc/tags`.