The FAQs are your friends. In addition, this post will show you where each of the pack's contents are found in PD in explicit detail.

Quote And I am pretty sure I have downloaded and installed at least 1 set/pack of add-ons from the zone.
With PD18 open, is there a way to determine just which add-ons I have?

The only packs from the DirectorZone that I am aware of are the Title sets. You can open PD18 and go to the Title room/Downloaded and those are what you downloaded and installed. You can go to the other rooms like the particle rooms and expand the Downloaded section too.

It may be better if you keep a diary of what you install on the pc. One can also use File Explorer to go to the downloaded folder on a pc and check the name, extensions, and dates of downloads from the DZ.

Good to know the degree of add-ons that are available - the listing at the link provided by opto doesn't show easily in the zone pages. And if you know/can remember which pack(s) you have got, following the links to the place where it is is not that hard. But it doesn't seem like there is a place in PD that will tell you just which ones you have.
Feedback item coming on.