A method of imaging the birefringence and optic axis orientation of a sample as a function of angle of incidence using conoscopic illumination and a rotating input polariser has previously been demonstrated on polymerized liquid crystals. In this work, we apply this technique to a planar nematic device with in-plane electrodes, which cause a twist in the director profile. The conoscopic images are compared with theoretical predictions based on a combination of a 1D nematic and an extended Jones optical method. The comparison allows values for the twist elastic constant K-22 and the azimuthal surface anchoring energy to be determined.