A number of measures are proposed as a microscopic means to analyse adsorption of gas on a surface and in slit pores under subcritical and supercritical conditions. Layer fluctuation of particle number provides us with information on where most of the mass interchange occurs, which can then be used as an indicator of the position of the interface separating the adsorbed phase and gas phase. The layer compressibility can be used to compare the adsorbed phase density with that of the bulk liquid. The layer isosteric heat provides an indication of the relative contribution of each layer to the overall isosteric heat. Finally, a histogram of particle number as a function of fluid-fluid particle energy is utilised to yield valuable information about the energetic structure of the adsorbed phase, for example (1) the number of neighbouring particles and (2) the evolution of the arrangement of particles.