Critical reading is an essential skill for participants in the civic dialogues and workplaces of the 21st century. Digital annotations are a novel data source for making visible these reading strategies. This work presents a step towards critical reading analytics by collecting digital social annotations and identifying student critical reading strategies across a diversity of college settings and texts. We synthesize prior research on critical reading to develop a data-driven, taxonomy of critical reading strategies. We then hand-code a sample for an exploratory analysis of student use of a digital social annotation tool.