The decay of electrical charge on a dielectric surface in nitrogen/C4F7N (Novec 4710, C4) mixtures is investigated using measurement of electric field via in-situ electric field-induced second harmonic (EFISH) technique. The charge is deposited on the surface of the alumina by generating a barrier discharge in the gap, and the amount of charge is determined from electrical current measurements and numerical modeling. For different admixtures (0%, 10%, and 50%) of C4F7N in nitrogen, the presence of surface charge is detected even 60 h after charge deposition. It is found that C4F7N admixture lead to a significantly longer-lasting surface charge, indicating a slower charge decay. Using an isothermal charge decay model, charge traps are identified for pure nitrogen charge deposition, which are in agreement with results found in the literature. Charge deposition in C4F7N admixtures leads to modification or creation of new traps with higher trap energies. The EFISH measurements are used to determine the C4F7N nonlinear hyperpolarizability tensor component (Formula presented). Direct comparison of the experimental results from two developed methods (EFISH and electrical measurements) and the numerical model gives a closer insight into the surface charge spread over the dielectrics, resulting in surface charge density estimation.