[Gabriel54]

I have never been asked such a thing, in the US or elsewhere. It would be on the customer to inform the staff of any allergies.

[devilbunny]

Five years ago it was a rarity (in fact the first place I can clearly recall being asked was a Five Guys, when I said I didn't want a bun - no gluten problems, thanks, just don't need the extra calories). Nowadays I'd say it's more common than not at full-service restaurants.

Southern US; I live in a modest-size metro of about 400k and spend plenty of time in bigger cities.