I take the point on religion - as how could you tell in the case of Irish terrorism that we suffered in this country who was Catholic of Protestant by looking at them.
Answer: You could tell by appearance to a reasonably accurate degree (about 80%.) The Irish catholics have almost black hair and heavier features than their Protestant counterparts (most of whom are of Scottish descent). There is, actually, quite a marked difference between Scottish Celts and Irish Celts!
However, you could be reasonably certain as soon as you knew the person's name. This was at the root of the 1969 "troubles", the feeling that if your name was Eamonn Murphy, Bernadette O'Connor or Sean Devlin then you were unlikely to get any sort of decent job in Ulster - on the other hand, if your name was John Stewart, Peter McMichael or James Brown then your future was reasonably secure!
......Indeed, many a Catholic or Protestant was targetted by sectarian gunmen purely based on appearance! The IRA and UDA paramilitaries (and their offshoots) weren't too particular about whether they got it right or not I recall!