With more and more systems and machines operating autonomously, the role of the operator is changing from “being actively in control” to “monitor and intervene”. Human Machine Interfaces (HMI) therefore need to be optimized so that they can efficiently be monitored by a human. We propose the Human Efficiency Evaluator (HEE), a software tool for (1) evaluating the impact of HMI design changes on the visual monitoring behavior of the operator and (2) to explore differences in understandings between a group of collaborating HMI designers or between HMI designers and their targeted audience: the operators. We describe the tool and highlight the model exploration capabilities of the HEE by reporting about two use cases: one in the maritime domain, in which the tool supported an HMI designer to get insights into human operators’ monitoring behavior, and one in the automotive domain, in which the tool was used to reveal differences in understanding between six Human Factor Experts about the impact of three HMI design variants of an Urban ACC.