The concepts of osmosis and diffusion are essential to biology, and student difficulties with them are well documented, along with multiple ways of assessment and diagnosis. We add to the literature by sharing how we used drawing and reasoning prompts with first-year undergraduate biology students to gauge learning before and after an osmosis and diffusion lab. We also share the development and validation of a three-point analytical rubric to score the keywords, student drawings, and reasoning prompts. The qualitative and quantitative nature of this exercise provides instructors with the unique opportunity to diagnose difficulties not only in introductory biology but throughout the undergraduate curriculum. Implementation strategies throughout the undergraduate curriculum are discussed.