The word see is talking about careful observation, rather than a quick glance. A true Christian is one who trusts in Jesus, who perseveres in that faith, who loves others, and who has the Holy Spirit living inside them. It is one of the works of the Holy Spirit to unite believers. Make every effort' he says. I know that there are members of this congregation who are not united in the way God wants them to be with other Christians.
Barriers build up and as the years go on they become more difficult to break down.
It is God's job to judge. We do not know enough and are not impartial enough to do it properly. When we judge others we step outside God's grace. We move into being put right by good works. Something that we cannot accomplish.