How can I check if two outlets are connected to the same electrical circuit?
To determine if sockets are on the same circuit switch off the circuit breakers one at a time to cut power to one socket and then check if the other one is cut too.

I have no idea what a "PowerLine" is and you haven't provided a link.
If it is a socket strip with a built in filter then it only protects devices plugged into it and not into the other sockets in the building.



Update:

It appears that you are asking about a WiFi extender that transmits data over the mains. Your fuseboard is single-phase so all circuits share the same live and neutral. The device should work.