How do you quantify/verify client synchronicity

[CarlosDerSeher]

Not sure if this is the right place to put this but here it goes @badaix ...
I am asking this because I picked up @jorgenkraghjakobsen work towards an esp32 snapclient. I've got a pretty satisfying solution in my repo at this point which works pretty well. Now when comparing to an Ubuntu client (v0.25) I always have some deviation which is in the order of 3-6ms,drifting around sometimes going up to almost 16ms. So I took another notebook, ran a snapclient instance and found that the Ubuntu clients also have a similar deviation. While 6ms isn't very much and probably nobody will be able to hear it, in your repo description you are stating sub millisecond synchronicity. So I was wondering if this is intended behavior or do I have something misconfigured? I often see messages like pBuffer->full() && (abs(median_) > 2): 2036 not sure if this is related.

in snapserver (openWrt on a mikrotik hap ac2) I use a buffer setting of 700ms, flac, 48000:16:2 and snapfifo to catch audio, all other is default. All clients connect using WiFi and are started with snapclient --player alsa -s 3 where 3 is the analog output

My method of measuring the deviation involves a 2 channel oscilloscope and cut open aux cables to attach the probes. This is my test signal: https://youtu.be/wo-wr_tRkCw
The clients are running for a more than 10 minutes sometimes longer in the test.

How do you verify synchronicity between clients?