As a first approximation they could be looking at what IP you connect from. unless it is coming from a known TOR exit point (and famously Facebook even operates an .onion address) or some VPN, that would give an indication what nation you reside in.

After that i guess they would look at what data you have given them.

The legal advice we got when we implemented things for this regulation was to go with geo-ip. And then people travel and you have profiles spanning across borders. (Or VPNs, or...)

Depends on the particulars of the company. If I know anything about risk-averse corporations, most of them would rather err on the side of considering you a Russian citizen.