You did not bring a backup
Look for bread, rice, fruit, or potato. Those show up in a lot of places, and one of them may already be a yes.
Find the ordinary food
If the only options clash with an allergy, skip the meal and use a safe food as soon as you can. An empty offer is better than a risky one.
Rice, fruit, potato, or bread, when wheat is fine, are the things to ask for. Plain, not sauced, until you know the sauce.
Say the allergy out loud
“No milk, so no cheese or yogurt.” “No wheat, so no bread.” Say it before the plate arrives.
A menu word is not a guarantee. If you cannot tell what is in it, leave it.
After you get home
The next meal can be the familiar food they missed. You do not have to make the outing up with a special dinner.
Next time, the backup goes in the bag. One food they already eat is enough to pack.
A sentence you can say
“Is there plain rice?”