In the currently merged (but not deployed) version of the feature, users who don't have CN rights see a dialogue box saying, "You are not authorized to preview banners, or your edit token has expired. [...]" An empty banner preview area and buttons are also visible.
I noticed that there is an awkward workaround, at the individual banner pages.
e.g. , if we click the link in the very top-left corner for "Preview last saved version" then we can see a preview.
Another related-task (duplicate?): T248217: Special:CentralNoticeBanners throws BannerPreviewPermissionsException on viewing