Carbon-encapsulated nickel nanoparticles (Ni@C) are of considerable interest for catalysis, biotechnology, and hybrid nanostructures. However, the distribution of carbon between the metallic core and the surrounding shell remains difficult to assess. In this work, we apply 13C NMR spectroscopy to directly probe carbon localization in Ni@C nanocomposites, complemented by reference measurements on hollow carbon shells. Two distinct 13C NMR signals are resolved. One line, centered at zero shift, is assigned to glass-like carbon in the shell. The second line, shifted to higher frequency, exhibits a strong temperature dependence and is attributed to carbon atoms dissolved in the ferromagnetic nickel core. The absence of splitting indicates that these carbon atoms occupy crystallographically equivalent positions, most likely octahedral interstitial sites. After annealing at 1273 K, the shifted line disappears while the magnetization increases markedly, confirming carbon migration from the core to the shell. Magnetization measurements reveal a distribution of Curie temperatures among different nanoparticle fractions, reflecting the inhomogeneous distribution of carbon interstitials within the nickel lattice. These results provide direct experimental evidence for a carbon solid solution within the nickel core and demonstrate the capability of 13C NMR to determine carbon localization in core-shell nanocomposites.