In this article the principle of instrumental (experimental) determination of the parameters of mode compositions of an optical signal propagating along an optical fiber is described. The measurement scheme is based on the use of a wellknown experimental setup designed to obtain the profile of the refractive index of an optical fiber. In addition to that setup, it is proposed to perform measurements for different angular positions of the investigated optical fiber, and for its different longitudinal positions. The proposed method is realizable in the case when the signal propagating through the optical fiber is characterized by increased intensity, which makes it possible to measure a nonlinear (Kerr) caused addition to the refractive index. It is proposed to calculate the desired mode weight coefficients by the method of probabilistic selection according to the developed method.