Well if they are doing something offensive report them. Otherwise declare war or give share map to everyone and it will appear for someone

Try to match people's writing and talking style with anon's and take guesses, however if you are not feeling like playing as a detective try sharing map with some people you suspect and see if they are on the "allies" tab of newspaper (or smthn like that)

I don't use the second thing much but caught a lot of anons with doing it the first way. Both will probably be painful to do on large maps though.

Edit: Send everyone an anon message telling them to reveal on the news or mail as your country and convince ppl to reveal.