I have a friend from Northern Ireland who insists he can almost always tell the difference between Protestants and Catholics just by looking at them. He claims he is almost always correct. He insists that you can usually tell if somebody is a Protestant if they have blonde hair (especially a certain shade of ginger-blonde, he says), blue eyes, pale skin and youthful looking facial features. And he says they're usually taller and bigger boned, thus, stockier.
Catholics, he claims are almost always dark brunette in hair, often blue-eyed, but sometimes brown-eyed too. Certainly more cases of brown eyes in Catholics than you'd see amongst Protestants. He also describes them to be darker complected, and sometimes even olive-skinned. Other noticeable features usually common amongst Irish Catholics he says are curly hair, chiseled, angular facial features; fine boned, slight builds, and a crazy look in their eyes. He says that Protestants will have a sleepy, gazing look in their eyes, but Catholics usually look very intense.
He also claims surnames aren't enough to distinguish who represents which community, as Catholics can be named Smith or Taylor as much as Protestants, and it is not uncommon for a Protestant to have a surname like O'Neill or McGuinness.
Given names are usually a dead give away, however. You can guarantee if you're speaking to a Willy or a Sam, he's a Protestant. But if he is named Pat or Kevin, he's most definitely a Catholic. Girls' names are a little trickier, but a Maeve will be a Catholic and a Victoria would be safe to say is a Protestant.