There is no perfect solution. However, factories are much more controlled environments than restaurant kitchens, so your odds are much better. Plus, these day

There is no perfect solution. However, factories are much more controlled environments than restaurant kitchens, so your odds are much better. Plus, these days, they require foods to have labels for potential allergens for things like using the same facility that makes other foods with those allergens (i.e. even if the product doesn't have $allergen, if other products made there do, they warn you). So if I had a condition like this, I'd feel much safer with factory-made foods/ingredients than with food from any restaurant.