Low engagement is a significant problem in youth mental health services. In clinical practice, assessment of a problem is integral to its successful resolution, and yet little is known about what methods providers use to assess treatment engagement or what indicators they use to infer high or low engagement. Thirty-five providers serving youth and families primarily through school-based mental health services in an urban setting participated in a workshop training on engagement procedures. Prior to the training, providers responded to open-ended questions about their own clients demonstrating high and low treatment engagement. Behavioral observation was the method reported most by providers to assess client engagement (94.3%), followed by spontaneous self-report by the youth or caregiver (22.9%). No providers reported using psychometric instruments to assess engagement. Types of engagement indicators used also differed according to level of engagement. For low engagement, 74.3% of providers reported using their observations of attendance, whereas for high engagement, 68.6% of providers described relying on observations of in-session participation or out-of-session homework. Few providers described using indicators related to the therapeutic relationship, client expectancies related to treatment, or client comprehension of the treatment approach and related goals. These results highlight opportunities for improving resources to support the assessment of engagement. Such resources could help providers know not only which clients need support but also where to focus that support and how to do so early enough to create a larger window of opportunity for change.