You can measure Brunauer–Emmett–Teller (BET). BET explains the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of np.
The standard method for calculating SSA is N2 adsorption by gas adsorption analyzer using BET isotherm at a relative pressure (P/Po) between 0.05 - 0.3 .