@jmo - I appreciate that if I present the info you will take action.
Currently I look at certain things:-
1) openings where country A launches all of it's armies at country B, it looks unusual as if they are not concerned about getting attacked by country C, he then doesn't get attacked by country C, not then or any time during the game
2) they never bounce each others moves
3) they never support each others moves, it looks like they are completely independent of each other but never accidentally clash
4) The games are not the smallest pots, nearly always above 50

buy-in.
5) The players play with each other sometimes, when they do they average and aggregates scores that are way better than when they play on thier own.