How? How can you tell if someone is in Christ or not?
We don’t claim to know if they are not in Christ - that is proving a negative, and like I said, we don’t know where the church isn’t, and indeed periodically have found Christians elsewhere who have entered into communion with us. We do know that the Orthodox are in Christ, and the Orthodox receive the Eucharist in a specific manner which varies depending on rite, but someone who was not Orthodox would not know the means by which we receive the Eucharist.
Also given that most Orthodox use a shared spoon, among other things, I feel confident anyone who does not believe in the real presence would be afraid to partake of our Eucharist.
Listen to you. What gives you the right to decide who is “authorized” to take communion? Who gives you the right to turn away the body of Christ outside of your church? Who gives you the right to decide who is a Christian and who is not?
1 Corinthians 11:27-34
But I could ask you the same question - what gives you the right to impose your views on other Christians? Why do you want our Eucharist, when you disagree with us on such important issues as the identity of the Blessed Virgin Mary as Theotokos, Transusbstantiation, Auricular Confession, the intercession of the saints, the veneration of icons, et cetera. Confessional Lutherans, Anglo-Catholics, Old Catholics, Assyrians, Roman Catholics, Eastern Orthodox and Oriental Orthodox have beliefs which are different from those of your church. We do not deny your status as Christians, or your potential salvation, but we do have different beliefs, including that that only those who are members of our churches and those churches in communion with us should partake (and we are not all in communion with each other, but the Assyrians and traditional Anglicans have the least restrictive rules, with the traditional Anglicans requiring agreement with the Nicene Creed and baptism as prerequisites, and the Assyrians also requiring belief in the Real Presence.