We characterize the director field in electroconvection patterns of nematic liquid crystals using polarizing microscopy. In particular, we study the transition from the simple roll pattern, that appears at the onset of convection in the dielectric regime, to the formation of the chevron superstructure, which involves a second pattern forming instability on a larger length scale. Our method provides a semiquantitative measure for the local amplitude of the twist mode, a soft mode which is invisible in the conventional experiment, but seems to be constitutive for many non-trivial electroconvection patterns. The results reveal the relation between convection rolls and twist mode and find it in accordance with a new theoretical model by Rossberg and Kramer [Physica D 115 (1998) 19]. According to this model, the interaction between twist and convection modes is the key prerequisite in a general mechanism that generates chevron patterns.