For me, a British Person, the answer is easy:
‘Dammit, I’ve just made a cup of tea, and it tastes like fish - don’t you just hate it when that happens?’
Anyone who doesn’t wince in sympathy would be immediately suspect.
What about the Cousins, and the rest of the world?
If I met them in a pub, I would ask if they’d care for a half of Guinness.
It does not come in halves unless you can prove you’re over 80.
You could just use the Blackadder method:
For a more specifically Scottish version of the test, I’d ask the question “Where does Nessie live?” looking not for the answer of “Loch Ness”, but the pronunciation of the “ch” in loch. Anyone who renders it as “lock” gets arrested and anyone who says “lake” is also charged with being utterly hopeless as a spy.
Of course, if we have someone who has spent years perfecting the accent, the question changes to “Complete the following sentence: Spout, handle…”
I’d ask for the song detailing the contents of a Whopper. I don’t eat the things, but will probably remember that jingle on my deathbed.
“Have you ever been to Mar-a-Lago?”
That jingle is for the Big Mac, not the Whopper! You must be a spy!
I would ask them a question about local politics in Australia, if they give any kind of reasonable or coherent answer, then they are clearly a spy.
“What are the 39 Steps?”
half a gwince… ha ha
the boomerang I think…
Tho I’m sure it’s the Big Mac jingle, and you’re right, I could SWEAR the whopper had a jingle song too that they tried to copy off McD. I , oddly, remember having a vinyl record of it. And I don’t mean Whopper Whopper.
I mean “Special orders don’t upset us,…”
Sing “Sleep Country Canada…why buy a mattress any where else…”
And wait for them to complete the jingle.
Hey, let’s schedule a day for you to come by the house. If that person shows up at my house, it must be a foreigner Spy.
Yes! “Hold the pickles, hold the lettuce, special orders don’t upset us!”
So, Sergei, would you care for chips or jacket spuds with your half of Guinness?
I’ve got the bugger cornered now.
I suppose it would depend on their age, but if they’re Irish of a certain age they’ll know all the lyrics to this ad.
… I’m not sure what this means 
