All-dielectric nanoparticles clusters have been attracted the attention recently due to their ability to sustain the specific collective modes excitation leading to new interesting effects such as Fano resonances. However, it is difficult to recognize such modes in the linear optical response of the structure without detecting the near field profile directly. In this work we propose a new method for detection of the oligomer eigenmodes excitation and numerically proof it for the nanoparticles cluster in the form of a trimer. These modes are indistinguishable in the linear optical response of the structure, but manifest themselves in the nonlinear response such as third- harmonic generation dependence on the pump polarization orientation. We belief that our method can be used for experimental identification of the eigenmodes of the complex all-dielectric structures such as oligomers of all-dielectric nanoparticles.