I remember when I was in college and toured Europe one summer I saw a guy who was obviously an American college student. Body language, clothing, facial features, the whole. I was w rong, 100% wrong. Humble ever since then.
I can spot Russian Jews such as myself fairly well. I actually did it on spring break last week before the girl could open her mouth. She just had the look.
If I see a blonde Slavic girl who is EXTREMELY gorgeous, I would put money on her being Belorussian. I have yet to see a Belorussian girl here in the states whose face I would describe as less than “beautiful.”
On the Charles bridge in Prague I could tell the milkbottle white, red-haired, 6’ tall guy coming towards me was Irish before he opened his mouth, but that is not exactly a challenge.
Irish men tend to be pale with either the Brendan Gleeson look, the Pierce Brosnan look, or the Colin Farrell look. They’re pretty easy to spot. Irish women abroad are harder to pick out without hearing.
Irishfella is twigged for British or Irish almost everywhere we go, I’m not. I look “latin” so can pass as local in Spain, France, Italy, Greece and Morocco…right up until I speak.
With Mango, Zara and H&M having stores in both the Uk and continental Europe a lot of British and Irish women are wearing the same clothing as European women…they just wear it differently. FYI the Gap has employed a team of British designers to design clothing especially for the British market. Preppy clothing just doesn’t sell as well here, so they have had to put a much more “fashion” twist on it.
Kiwis are easy to pick out, IMHO. A LOT of the ones you run into overseas will be wearing either Polarfleece jackets, (probably Kathmandu) or some sort of Greenstone Pendant with a Maori design. Anything with a Silver Fern on it is also a dead giveaway as well.
Like TheLoadedDog says, Americans tend to stand out a mile away here in Australia; not only with the “baby-face” skin but also the fact you lot all seem to have teeth like Game Show Hosts. 
The vaguely lost look tends to give it away as well- We get American exchange students in at work a fair bit and they often look slightly lost and bewildered, like a moment ago they were in Seattle or Des Moines or somewhere like that, and suddenly they’re on the other side of the planet and Rod Serling is about to step out from behind a tree and announce that they’ve just undertaken a trip into The Twilight Zone.
The Brits tend to carry themselves very much like Australians, so it’s hard to pick them out of a mixed-nationality line-up without clues like Arsenal T-shirts or knotted handkerchiefs on their heads. 
Well, there is a margin for error. I mean, they could be Italian, or Portuguese, or from Southeastern France…
but yeah, I’ve gone into a room, looked at all the 100+ people there, spotted two groups that had to be the Italians and the Spaniards and put my money correctly on which one was which.
I can also identify different locations within Spain. Not always, but the only time I guessed someone as being from Soria and he said “uh, noooo”… it turned out his parents were. Well, D’UH! There aren’t a lot of other places where you get guys of his size and coloring.
I think Irish people (full Irish) have a certain look to them. I think it’s in the cheekbones and the way the eyes sit on the face. I can pick them out a lot. This might also come from knowing so many of them that it’s become familiar, just like if someone knew a lot of Chinese people they could probably pick them out against other Asian persons of similar appearance.
Sometimes people assume I’m 100% Irish (because of the red hair) and I always say my face doesn’t look Irish at all. If I point out some real Irish people they can see what I mean.