The disclosed device for verifying a hip-knee-ankle angle includes a mounting base having a planar abutting surface adapted for direct abutting against a resected surface on a distal femur, and a first inertial sensor in communication with a computer assisted surgery (CAS) system to determine an orientation of the mounting base and to digitize a mechanical axis of the femur. A visual alignment guide element is pivotably mounted to the mounting base such that the angular position of the visual alignment guide element is adjustable so as to be visually aligned with a mechanical axis of a tibia. A difference between orientations of the mounting base and the visual alignment guide is calculated by the computer assisted surgery system to determine the hip-knee-ankle angle. The visual alignment guide may include a second inertial sensor and/or a laser emitting element.