They look identical to private messages, basically, except instead of an envelope watermark in the right gutter, there is the “group” glyph:
There is no visual difference from the topic list or the category selection area, however. It’s all at the topic level.
I agree that PMs and private topics need to look visually quite distinct. But as of right now it’s all about the right gutter watermark.
Of course you have seen what staff actions in a topic look like; just find any closed/pinned/archived topic and look for the post event marking it – background color.