Yes, you can preview your non-activated galleries before making them public to your clients. Here's how you can do that:
From the Gallery List
From the main menu, click into Photos » Galleries.
Locate your inactive gallery and click on the settings icon (which is shaped like a cog).
Click on the Copy the Preview URL option from within the drop-down menu.
Once the URL has been copied, you can then paste the URL into your browser's address bar to view a preview of the gallery.
From the Gallery Settings
From the main menu, click into Photos » Galleries.
Locate your inactive gallery and click on the name of the gallery.
Once the gallery's settings have displayed, click on the the View Gallery Preview link.
Can't find the preview options?
If the preview options aren't displaying for you, that could mean either of two things:
The gallery is currently viewable to your clients. If the gallery is currently active (it's status is set to "Allow Visitor Access"), the preview links will not be available.
The gallery has expired. The preview links are only available for galleries that are not currently expired.