Natively, I'm not sure you can. Using an admin token you might be able to get some of the information usually displayed in the admin panel, but as a guest or regular user you would have to guess by crawling data from the API.
If you use an extension like this one the information will then be made available some place https://discuss.flarum.org/d/28403-afrux-forum-stats-widget I'm not sure where exactly but I suspect it will be injected in the GET /api
payload.