The extent of adsorption at a liquid interface can be evaluated using the surface tension concentration data and the Gibbs adsorption equation.   The microtome blade method is used to determine the weight and molal concentration of an interface.  The method involves attaining a one square meter portion of air-liquid interface of binary solutions using a microtome blade.
Another method that is used to determine the extent of adsorption at an air-water interface is the emulsion technique, which can be used to estimate the relative surface excess with respect to water.
Additionally, the Gibbs surface excess of a surface active component for an aqueous solution can be found using the radioactive tracer method.  The surface active component is usually labeled with carbon-14 or sulfur-35.