Interactive systems including multiple interaction devices and surfaces for supporting the collaboration of a group of co-located users are increasingly common in various domains. Nevertheless few collaborative and multimodal interface specification notations are proposed. As a first step towards a notation for specifying a design solution prior to its software design and development, we adopt an empirical approach. We applied and compared four existing notations for collaborative systems by considering a case study, namely, a system for supporting informal co-located collaboration in hospital work. This paper reports the conclusions from this empirical comparison.