In situ undrained shear strength measurement devices such as the vane shear test, cone penetrometer, and full-flow penetrometers (e.g., T-bar and ball) have been increasingly used as a part of geotechnical site investigations for soft clay deposits. Previous attempts to visualize the shear failure surfaces on these devices included either significant disturbance of the soil sample (permeated with resin and thin-sectioning) or involved methods of shearing along a transparent surface creating boundary effects. This paper described the use of non-invasive testing procedures to determine the failure surface of several in situ shear strength measurement devices in a soft clay surrogate (LAPONITE RD, a trademark of BYK Additives and Instruments). A series of tests were performed using two sizes of rectangular miniature vanes, two types of full flow penetrometer tests (T-bar and ball), and a cone penetrometer in LAPONITE RD samples. Laser planes were used to illuminate particles in the transparent material during shearing. Particle tracking techniques were used to visualize the failure surfaces and flow mechanisms for each device. Experimental results were compared with theoretical and numerical simulations of the failure surfaces and flow characteristics by previous researchers. The present work contributed to the understanding of undrained shear strength testing devices by an in situ assessment of the evolution, geometry, and structure of the failure surfaces and full-flow mechanisms.