The mark test is a test where a mark otherwise undetectable by touch or smell etc. is placed on the face (or another part of the body invisible without the aid of a mirror).  If the subject, after viewing themselves in the mirror, moves their hand (or another part of them if its an animal) up towards the mark on their own body, they are said to have self-recognition.  Children usually start being able to pass the test (but not always and not perfectly) after about 18 months.