The absence of a model to predict near wall viscosity of complex suspensions instigated an investigation for a new method to determine the wall shear stress. If the inner wall of a flow model is covered with a highly flexible gel layer, the local wall shear stress will deform this gel layer. Through the known properties of the gel layer, the measured deformation can be transformed to the wall shear stress. To measure the deformation of the gel layer, speckle pattern interferometry was applied. The performance of the developed speckle apparatus was evaluated using a well-controlled benchmark experiment, and a resolution of 50 nm was achieved. The deformation of a gel layer was measured in a two-dimensional rectangular duct, using a Newtonian and a non-Newtonian measuring fluid. The wall shear stresses were measured as a function of the flow rate and compared to theoretical predictions, and the results demonstrated the potential of the method.