For Apple TV 4K, if an iTunes movie has 4K available, but possibly due to my other Internet usage, the speed required (25Mbps recommended) is or is not possible. Then how can I tell which resolution the movie is playing at currently? (whether it is playing at 4K or not).
When the iTunes movie is playing, drag down from the top of the remote to the bottom of the touch area. This will show the info panel from the AppleTV with information about the movie.
At the top you'll see small icons representing the currently played quality. For example "HD", "4K" or "SD". It might also display "HDR" and similar.
For the frequency, the best way is to check the TVs "Info" panel (or whatever it is called on your TV). This will show the resolution and frequency that the AppleTV is currently playing at. When you have frequency matching enabled (which is the default) this will match the source frequency of the movie.
Note: This method is not suitable for determining resolution when playing back from for example YouTube where content might be 240p or similar low resolutions. In those cases look at what the Youtube app tells you.