Pardon my ignorance, but I'm curious how you knew they were Roman Catholics and not Orthodox. Did they say so and then present anyway, or is there a particular "way" Orthodox present for Communion that you knew right away they weren't following?
I wonder too. But, having been in an Orthodox Church and a Roman Catholic Church, I have seen that they do things quite differently. REAL DIFFERENT.
"Pardon my ignorance, but I'm curious how you knew they were Roman Catholics and not Orthodox. Did they say so and then present anyway, or is there a particular "way" Orthodox present for Communion that you knew right away they weren't following?"
You're pardoned. After all, how would you know the secret password and handshake?
Seriously, most Orthodox parishes are quite small and the priest knows everybody who is a regular. Those few Orthodox who show up but are not regular a) carry themselves in a way and do certain things during the Liturgy, like the way we cross ourselves and kiss icons which mark them as Orthodox and b) announce their baptismal name when they reach the priest, no matter what language the priest asks the name in. Any doubt and the priest simply asks if the person is Orthodox. I can tell you that we almost always know when someone who is not Orthodox is in Church, unless of course they are catechumens in which case they wouldn't present themselves for communion anyway.
Ask them to cross themselves and then see where their trained instinct takes them. We go right-left, catholics left-right.