Co₂FeSi is predicted to be a half-metallic ferromagnet with an extraordinary high magnetic moment and Curie temperature. However, a low tunnel magneto-resistance ratio, a lower spin polarization and a lower magnetic moment were experimentally observed in thin film samples. Consequently, thin Co₂FeSi films of different groups were studied using spin-echo nuclear magnetic resonance (NMR). NMR probes the local hyperfine fields of the active atoms, which strongly depend on the local environment. NMR is thus able to reveal the next neighbouring shells of the ⁵⁹Co nuclei in the Co₂FeSi thin films. As expected, our NMR study shows the main resonance line corresponding to ⁵⁹Co nuclei in the L2₁ environment but also additional resonance lines at the high frequency site of the main line with a spacing between adjacent resonance lines of 32 MHz. The additional resonance lines correspond to ⁵⁹Co with more Fe next neighbours than expected for the L2₁ type ordering, which is interpreted as the formation of an off-stoichiometric film yielded by sputtering from a stoichiometric target. The analysis of the NMR data and a comparison with off-stoichiometric bulk samples reveals the composition of the thin films to be about Co₂Fe(Si_0.92Fe_0.08). This off-stoichiometry might explain the observed deviations from the expected behaviour.