How to tell the latency to Airplay2 display ?

Hello,

I would like to be able to measure the latency between iphone and airplay2 display.

To explain why I need this:
My iPhone App is playing a video which can be displayed on a larger screen via AirPlay.
While playing the video, the user can tap on the iphone and interact with the items displayed on the screen.
To be able to sync between user commands and the displayed video, I need to have a rough estimation about which frames are being displayed at any given time (so I can tell what the tappings are related to).

I noticed that average latency is about 0.4 seconds, but I dont want to just assume that, I prefer to know the current latency for each user in realtime.

Thank you,