Or any other social function.
If they have a foreign-sounding name, but say they are from/ live in the town of the country you are currently in, insist on repeatedly asking “Yes, but where are you from originally?” Keep doing this until they start looking confused or find an excuse to move away.
Tell them about your former teacher/ classmate/ neighbor/ the shopkeeper next door who was from there and the strange things they used to do that you “just couldn’t understand”. Then look expectantly at the guest, signalling the need for an explanation.
Insist that they must know some dish or food from their native country, even if they have already denied this knowledge several times.
If they are actually from somewhere else, contradict them about their country in front of everyone. “I went to this two-hour seminar three years ago, and what you say isn’t correct!”
Start a heated monologue about said country’s politics, trying to make it a dialogue. This one is always a winner.
Refuse to accept they don’t like the alcoholic beverage every single member of the population (children excluded) of said country supposedly drinks. Harp on about this for a while and declare it “unbelievable”.
Tell them they can go mingle “over there” with the other people from said country.
Ask them how to pronounce certain words in their language which you think you know and then stubbornly don’t get it.
Ask them how come they know the language of the country they currently live in, even after they said they studied in current country.
Name one person you know, or just pick a name, supposedly from the native country, and ask your guest if they know this person.
Voila, mission accomplished.