While it doesn’t solve the issue of not knowing a feature is there in the first place, if there’s something you’d like to do in an app, you can get a long way by hitting “Help” in the menu bar and typing it into the search field. If there are any matches for it you’ll immediately know you can do it, and it will open up the menu and show you exactly where it is (and the keyboard shortcut, if there is one).

This is the primary way I look for power-user or other non-obvious features in new Mac apps.