YouTube videos from a free account, even those listed as Atmos, are downgraded to 2.0 PCM to save space.  You can go here:

https://www.dolby.com/ja/about/support/guide/downloadable-dolby-atmos-trailers/

Or here:

https://www.reddit.com/r/sonos/comments/1ggy9jd/dolby_atmos_natures_fury_trailer_and_test_tones/

To get actual Atmos trailers.  Download to a USB drive and connect to your streaming device or TV to run them. 

Do you subscribe to any music streaming services that offer Dolby Atmos? e.g. Apple Music or Amazon Music? If you play an Atmos or Spacial track, the Sonos app’s Now Playing screen will show the Dolby Atmos symbol if all is working well. (If you search for Dolby Labs you should find some Dolby Atmos albums of test sounds that you can play too.)

Also, do you subscribe to a TV streaming service at a tier that provides Dolby Atmos? If so, play an Atmos show or movie, and the now playing screen will again show Dolby Atmos if all is working as it should.

Note that much might also depend on the TV’s sound settings being correctly set up to either pass through the Dolby Atmos signal from an external device (like a cable box, Firestick or Apple TV) or to output from the TVs built-in apps in Dolby Atmos. 

Content MUST be in Dolby Atmos for it to work. It doesn’t covert stereo, for example, to Dolby Atmos.

I tried using atoms music from Apple Music, and I also tried an Atmos movie from Apple TV 4K .  I just can’t tell if it’s working or not.

Here's a folder with tests for everything: Dolby Atmos 5.1.2, 7.1.2, 9.1.4 (This for your Arc Ultra), etc.

https://drive.google.com/drive/folders/1JxmeedtAtgmoafXv9rroiDOS2vEX7N4b

You need to put them into a USB and play in your TV :)

This is great!  Which one do you recommend I use to test the Atmos of just the arc ultra and no surrounds.

For only test the arc ultra without the surrounds speakers you need to disable it a moment because if not all speakers will spund for better sound, and the best test is the one who the name is "Dolby Atmos E-AC-3 9.1.6.mp4" I can't found a exactly version of 9.1.4 but it works :)

Any of the above will work, just disable / unplug your surrounds.

If you look at the Now Playing screen in the Sonos app, the audio format will be shown.  If you are getting Atmos, it will display as Dolby Atmos.

OK, I confirmed using the Dolby app that I am streaming in Dolby Atmos. That’s what the app is seeing. However, I am just disappointed in the lack of separation between the sound. I thought it would be more akin to a 5.1 surround system or better. I think I was just expecting a little too much from the speaker because I hear nothing from the side or above

Much depends on the source, just being Atmos isn't a promise of good sound. I have found many tracks that sound horrid, no low end, no separation or worse odd positioning.

Find a good source and you will likely be impressed, be it a 300 pair or a full TV set.

If you have a firestick, their audio test does a great job on Atmos.

Try to watch content in Apple TV+, they have all their content in Dolby Vision/Atmos and sounds really good or in Netflix and Disney+ have some content that sounds spectacular, like Stranger Things. But yes, as Stanley said not all content is good by being in Atmos, specially in the music I think.

There are numerous threads in the forums here with people very dissatisfied with separation from the Arc Ultra. The feedback given in one threads was Sonos saying their, super, duper, world class, sound engineers say that is the way it is and there is nothing wrong with that since it is intended to be this way. Guess that's what you get after paying premium price for supposed premium product.

Many people have simply suggested allowing us a center channel slider that would help separate the channels better. Mainly since the center channel is noticeably louder than virtual fronts.