In this paper, we report the first local magnetic study, performed by magneto-optical Kerr effect (MOKE), of supercrystals made of magnetic nanoparticles. The supercrystals are characterized by a film morphology and contain several hundred layers of 7.7 nm-Co NPs, ordered in a face-centered cubic (fcc) superstructure. Conversely to the standard superconducting quantum interference device (SQUID) technique, which provides only a global measurement over an entire sample, the utilization of MOKE enables magnetic measurements of individual localized areas of c.a. 30 µm x 30 µm, from which we can draw conclusions regarding the uniformity of the magnetic properties of the sample, and therefore on the high uniformity of its structure. The latter conclusion is supported by Monte Carlo simulation performed on a model system composed of spherical particles interacting through the dipole-dipole interactions (DDI) and exhibiting a uniaxial magneto-crystalline anisotropy energy (MAE). In addition, by combining these results, with those obtained by SQUID and by grazing incidence small-angle X-ray scattering (GISAXS), we show that magnetic and super-structural properties of supercrystals can be accurately determined.