When single wall carbon nanotubes are mechanically or chemically cut, the ratio between opened and closed tubes after treatment is difficult to determine. We propose a new way to characterize this using rare gas adsorption. We compare theoretical calculations and experimental results performed on both closed and opened nanotube bundles. Plateau height analysis allows extracting the tube density in the bundles. Secondly, upon exposure to Kr or Xe, the adsorption isotherms obtained for closed and opened tube samples differ substantially due to the presence of different adsorption sites and thus can help to evaluate the efficiency of the opening method.