This is my second "I feel like a complete idiot" question of the week, so thank you for your patience.

How does one find an app-id, e.g., for setting up window rules in my window manager (River)? For example, if I'm using Nautilus as a file manager and I wanted to have the Nautilus Previewer window float by defining a River WM rule, I can do every bit of that trivially, other than identifying the app-id. (In this case, I believe it's org.gnome.NautilusPreviewer, but I'm looking for a general case.) Please note this question is about Wayland and not X.

I dropped into GNOME and viewed active windows with Looking Glass (lg), but that seems like a silly workflow just to ID a window.