Determination of compositions in multiple quantum well structures is a critical issue. Until now only few experimental description of determination of concentrations in the nonpolar m- or a-direction in III-nitrides by means of X-ray diffraction has been published. Using the example of GaInN/GaN multiple quantum well structures we present different methods to determine the indium concentration. After presenting the relevant elasticity relations we adopt a step model introduced by Segmüller and Blakeslee to nonpolar directions. This model allows a simulation of symmetrical scans. We will discuss the possibilities and also the limitations of this model. Additionally, we present a method to determine concentrations by using the peak position of the zero order superlattice peak. Finally, we compare the methods using samples grown on m-plane SiC and bulk m-plane GaN.