What the law requires
Regulation (EU) No 1169/2011, Article 9(1)(c) makes it mandatory to indicate every ingredient listed in Annex II that is used in a food and is still present in the finished product. Annex II lists 14 groups: cereals containing gluten, crustaceans, eggs, fish, peanuts, soybeans, milk, nuts, celery, mustard, sesame seeds, sulphur dioxide and sulphites above 10 mg/kg or 10 mg/litre, lupin and molluscs. Under Article 44 this information is also mandatory for food offered to the final consumer without prepackaging, such as the dishes of a restaurant or a food truck, and each country may set how it is given. Regulation (EC) No 852/2004, Annex II, Chapter IX adds that equipment used for these allergens must be cleaned and checked before it is used for food that does not contain them.
Food sold without packaging must still come with information on the 14 allergens listed in Annex II of Regulation (EU) No 1169/2011. How you give it, for example on the menu, on a sign or orally, depends on the rules in your country.