This question seems to be a bit of a bone of contention with Parks in his book Italian Ways. In the course of his train travels through Italy, he is constantly irked when Italians speak to him in English before he's even opened his mouth. He doesn't understand how they know he's not Italian.
Where I sometimes go to write, in the library in Trinity College, I see tour groups through the window of all nationalities queuing up to see the Book of Kells. My guesses as to where they're from are usually right. It comes down to what they are wearing, what they use to carry their belongings, whether the men have beards and/or sideburns, how they wear their hair, what accessories, how they walk, how their facial muscles move when they talk, how they respond when the group leader is speaking, how they interact with each other… The list goes on. It's not foolproof, but the clues are usually there, in the language of the body. Parks understands this language too:
The nose is the dominant feature, long, thrust forward, slim, very slightly hooked. The eyes are large and very carefully defined with make-up, the eyebrows plucked in high arches. The forehead slopes back at quite a marked angle, accentuating the nose, and the thick raven hair, which is firmly gathered and swept back, is held tense and tight by a headband and three long wooden skewers, poking up and out at spiky angles…
His wonderfully observed description of a typical southern Italian woman's face is that of a fully tuned-in writer. Perhaps, just as it's easier to solve someone else's problems than one's own, easier to spot other people's faults, it's also easier to detect characteristics, and sometimes national characteristics, in others than to define them in ourselves. It's forgivable that he can't define his own Englishness as succinctly.
What I don't understand is why it should bother him — what's wrong with being taken for what he is, an Englishman who lives in Italy?