Non-suicidal self-injury (NSSI) is prevalent among adolescents and a sign of suffering. The behaviour is often associated with shame, and professionals are unsure of how to inquire about it. The implication is that many young people do not tell about their NSSI. This review introduces a short and easy-to-use screening tool called "The 3 F's". By means of "The 3 F's" the form, the frequency and the function of NSSI can be uncovered. The screening tool can be used for systematic detection of NSSI in order for professionals in a respectful way and with a validating attitude to show the adolescent that they wish to understand the underlying cause of the self-injuring behaviour.