We report a combined experimental and theoretical study to ascertain carbon solubility in nickel nanoparticles embedded into a carbon matrix via the one-pot method. This original approach is based on the experimental characterization of the magnetic properties of Ni at room temperature and Monte Carlo simulations used to calculate the magnetization as a function of C content in Ni nanoparticles. Other commonly used experimental methods fail to accurately determine the chemical analysis of these types of nanoparticles. Thus, we could assess the C content within Ni nanoparticles and it decreases from 8 to around 4 at. % with increasing temperature during the synthesis. This behavior could be related to the catalytic transformation of dissolved C in the Ni particles into graphite layers surrounding the particles at high temperature. The proposed approach is original and easy to implement experimentally since only magnetization measurements at room temperature are needed. Moreover, it can be extended to other types of magnetic nanoparticles dissolving carbon.