Go to the gsow.net website and search by member. That will tell you if the player is in an active squad that wars. If they are not in a squad or in a dead squad that does not war, they will not show up. That's the best chance, unless you want to join the various LINE, Discord, and other chat groups to try and track them down. If they don't come up, it most likely means they may have quit the game or are not in a squad.
Squad Level: 50