Nobody can.  Hi-Res audio is pure snake oil.

I believe with apple, if you play within the sonos app it will be lossless, if that is what apple is providing. If you use airplay 2, it likely downsamples to 256 which is common for airplay 2. Sonos is now one of the few ways to actually easily play lossless with apple. Bluesound cannot. If you use wireless earbuds on your phone,  you cannot. If you use a DAC connected to your phone and wired headphones you can. 

Could you tell, if I would be able to stream lossless Apple Music via Sonos Port’s Digital output to my receiver? Thanks in advance. 

 You will not be able to hear the difference between 16/44 and 24/48 ( the Sonos limit ).  So if Apple is showing Lossless that’s all you need to see.  If there was a difference between 44 & 48 Sonos would not be able to resolve it.  You would need to spend considerably more to hear any difference if there was actually a difference.

 If you are concerned about “stair stepping” get a system that can play DSD.

Just switched over to Apple Music due to Google's price increase shenanigans. Very noticable difference in fidelity vs lossy sources if the speakers are up to it.

Ears picked it up immediately on a couple of my audio systems, including the Arc/Era300/sub group in my living room. Love how well Sonos handles music on their multimedia devices. A soundbar surround system just shouldn't do this well with music sources, but this one really does. It's so wrong but it's so right! 

 

I push that question, because I‘m asking myself the same and there was no reply to @Yenzi s post. 

Sonos App for the described situation still just shows „Airplay“ without any further information about stream quality. 
I tried the same using Amazon Music UHD stream from AM app via Airplay to Sonos and I‘m not sure what in detail Airplay is submitting. The max possible 24/48 capability of Sonos would be nice.

That said i‘m also wondering if Sonos will support Atmos via Airplay to its supported Atmos speakers after Apple this fall introduces Atmos via Airplay. 
 

Does anybody know more about lossless or atmos via AirPlay? Especially regarding to Sonos?

 Why use AirPlay when you can use the integrated Apple Music service in the Sonos app?  I’m sure there are reasons to use AirPlay.  I’m just not understanding what they are.

I‘m asking primarily because of technical curiosity.

But often people prefer using the native app of a streaming service. And if there’s no direct control option like e. g. Spotify has, using Airplay is a welcome alternative. 
However my question is, if Sonos might support lossles and atmos via Airplay after Apple hopefully soon (announced for this fall) is going to support it. 
At the momemt imho probably the 16bit/44,1kHz limit still exists. 

 Got it.  I have a question about AirPlay.  The only time I tried it as a test I had to crank up the volume on the phone to a very high level to get the volume I’m used to.  Then when I quit using AirPlay the volume of the Sonos system I was using it on was way too high.  I caught this condition and lowered the volume in the Sonos app, but if I had not noticed the excess volume I would have gotten a nasty surprise.  Is this normal AirPlay behavior?

Could it be that‘s a very long time ago?

If I remember correctly at the very first time of Airplay on Sonos volume control via Airplay and Sonos were not synced. But now it is and changing on one side also changes the other.  I just think one single Airplay volume step is about 4 Sonos steps and for me a little bit inaccurate. 

A way to tell is to compare the amount of data flowing from phone to the master Sonos device, versus flowing to the Sonos device when not using AirPlay. My WiFi network is able to show these numbers.

From Apple Music to a Move, showing Lossless in the Sonos App, I see of the order of 1.7-1.8Mbps inbound and, when linking a Beam and its surrounds into the mix (because the Move doesn’t hook in the same way) I see that same amount of data flowing out from the Move to the Beam. This fits with Apple’s quoted typical 36MB for a 3 minute song (1.6Mbps)

If I stream from Apple Music to my phone, configured for Lossless when WiFi Streaming I see a similar amount of data flowing (but the phone seems to preload the track as quickly as it can).

If I AirPlay from phone to AppleTV, I see the preload happening with data flowing directly to the AppleTV. That is, the phone handed off the stream entirely, and the AppleTV retrieved the Lossless version of the audio.

If I AirPlay from phone to Beam, I see data inbound to the phone, and a much lower data rate outbound from the phone and inbound to the Beam- maxing out at 250kbps, but generally a lot lower. From that I’d conclude my phone is re-encoding to AAC for AirPlay and not handing off the stream.

For a more accurate analysis I’d look closely at actual total bytes sent and received, but it seems fairly obvious that AirPlay to Sonos is re-encoding - and that fits with what others have said in the past about its capabilities and behaviour.

Thank you @jrguk … in the meantime I found out that Apple currently just supports Atmos via AirPlay stream to its own Home Pod devices. That definitely fits to your test results.