![](https://i.sstatic.net/YGr8j.png) Final solution not requiring to be root, and accessible from a non-interactive shell in a automated way :

 -`XAUTHORITY` :

 ps -u $(id -u) -o pid= |
 xargs -I{} cat /proc/{}/environ 2>/dev/null |
 tr '\0' '\n' |
 grep -m1 '^XAUTHORITY='
- `DISPLAY` : 

 ps -u $(id -u) -o pid= |
 xargs -I{} cat /proc/{}/environ 2>/dev/null |
 tr '\0' '\n' |
 grep -m1 '^DISPLAY=:.*$'

Based on [this](https://superuser.com/a/975536/68500)