To find out, for a given holidays set, which recurring holidays are referred to by other holidays via a conflict rule, use the corresponding button located on the holidays sets toolbar, as shown below.
The search lasts a few seconds and, if any holidays refer to holidays in the current set, a dialog appears displaying the list of holidays within the set that are referred to by other holidays. These references come from holidays that refer to the displayed holidays through conflict rules.
These links are detailed into from same set and from other sets. This distinction is useful when you are about to delete an entire Holidays Set; any links from within the same set are not a problem as these holidays will also be deleted. Links from other set are, however, a concern as links (and the rules that use them) will be broken if holidays listed in this dialog with outside links are deleted.
See also: finding referrers to a specific holiday rule.