We have developed high resolution x-ray diffraction techniques to determine the order parameter in nominally lattice-matched In_xGa_1-xP epitaxial layers deposited on GaAs by metal organic vapor phase epitaxy. Earlier work on ordering in semiconductor materials relates the width of the x-ray diffraction peak to the size of the ordered domain, but does not predict the order parameter. In this study, a matrix of samples with different amounts of ordering was obtained by varying the substrate miscut and epilayer growth temperature. A combination of ½(115) and (224) reflections in double axis was employed to compare diffracted intensities of standard and "ordered" reflections with approximately the same beam footprint. Only one strong variant was observed in each case, consistent with the presence of a miscut. The composition was also determined by x-ray diffraction and combined with luminescence measurements to provide an independent measurement of the degree of ordering. The values for the ordered parameter obtained were in agreement with those from diffraction. Dynamical simulations of CuPt-ordered InGaP were also used to refine the order parameter determination. Initial triple axis diffraction reciprocal space maps from the "ordered" reflections also indicated that the peak width is not always related to a domain size but rather to misorientation among the ordered domains.