The invention is related to a method for qualifying and quantifying carboxyl groups on the surfaces of a silicone hydrogel contact lens. The method of the invention comprises the steps of: (a) obtaining silicone hydrogel contact lenses each of which comprises a silicone hydrogel lens body (i.e., bulk material) and a coating thereon, wherein the silicone hydrogel lens body is obtained from a lens formulation free of any polymerizable component including one or more carboxyl groups, wherein the coating comprises a polymer having carboxyl groups; (b) immersing a given number of the silicone hydrogel contact lenses in a fixed volume of a C1-C4 alcohol solution of a positively-charged dye having acetate as counter anion at a given initial concentration for a time period sufficient to stain the silicone hydrogel contact lens; and (c) determining the concentration of carboxyl groups per each silicone hydrogel contact lens and/or the thickness of the coating on the silicone hydrogel contact lens.