Scales for measuring organizational justice are available but is there a separate scale for measuring injustice? Or the lower scores on justice scale would mean injustice?
Nyarko et al (2014) Organizational Injustice and Interpersonal Conflict on Counterproductive Work Behaviour. European Journal of Business and Management Vol.6, No.21
See p.119.section 2.3.2. This is as you suggest, the lower scores represent organizational injustice. The paper below is the one that the authors used.
Colquitt, J. A. (2001). On the dimensionality of organizational justice: a construct validation of a measure. Journal of applied psychology, 86(3), 386.