This case study presents the development of a picture-based interview protocol designed to assess STEM identity among Black elementary students participating in a community-driven environmental justice program called 'Engineering Roots'. Created in response to a lack of developmentally appropriate and culturally responsive tools for younger learners, this protocol incorporates visual prompts and open-ended questions that allow students to reflect on their interests and self-concept in relation to science and engineering. Pilot-tested during a six-week STEM camp in Summer 2024, the protocol demonstrated its potential to elicit rich, actionable data about students' STEM identity development. This work contributes a novel tool to the field, providing educators with a meaningful way to assess and foster STEM identity in young learners, particularly in communities underrepresented in STEM fields.