Variability is a central concept in software product lines (SPL), but there is no instrument that measures the ability to think about software variations. This gap limits the assessment of skills, the comparison of profiles, and the evaluation of educational interventions. This article analyzes whether it is possible to define such an instrument. We present the Test of Variability Thinking (TVT), an exploratory visual test grounded in the classic SPL framework and based on a technology-neutral physical domain (Playmobil). The TVT is inspired by existing computational thinking tests and aims to capture the basic ability to perform tasks related to variability without presupposing prior technical knowledge. The basic idea is that variability thinking is a new dimension of computational thinking and requires specific instruments to be developed and evaluated. We describe the principles guiding its design and the construction of the items. A preliminary workshop with 10 researchers with varying levels of SPL expertise yielded initial feedback that shaped the test structure and informed the selection of item types. Then we had a focus group with seven software engineering students, which helped us further refine the phrasing of the items and improve the visual elements. Finally, the test was administered to 88 fourth-year software engineering students as the first step of its validation process. The resulting version offers initial evidence of feasibility and motivates future work on validation and empirical evaluation.