There is no official registration or certification process for assistance dogs in the UK so there is no legal ‘proof’ that you can ask for from someone who is accompanied by an assistance dog.
People with assistance dogs trained by an Assistance Dog UK (ADUK) member organisation are issued with an ADUK ID Booklet containing details about the dog and the charity that trained it. Some may use the Assistance Dogs International App on their phone to show that the organisation that trained their dog is accredited.
However, not all disabled people who rely on an assistance dog will have trained their dog through an ADUK charity so they may not carry specific ID, nor are they required to by law.
Assistance dogs might also be recognisable by their jacket, harness, lead slip or bandana but there is no legal requirement for them to wear these. Some people may choose not to use these and legally, they are within their rights.
ADUK recommends treating each person as an individual and if they assert that they are disabled and that their dog is a highly trained assistance dog, and the dog behaves as expected, it may be unlawful to deny them access. If it becomes evident that the dog is not behaving in line with the list of expected behaviours outlined by the Equality and Human Rights Commission which are listed below then refusing, or revoking access may be justifiable.