Despite of the advances achieved in the past years in order to design more natural interfaces between intelligent systems and humans, there is still a great effort to be done. Considering a robot as an intelligent system, the determination of some typical interaction situations is an interesting ability to achieve. This paper shows a fuzzy system that allows the visual detection of possible interaction demands and the shaking or nodding of the head. To achieve this objective, the robot has first to carry out the detection and tracking of the people using the stereo vision system. Then, the level of interest of a person to interact with the robot is calculated by analyzing his/her position and the pose of his/her head. The head pose is estimated in realtime by a view based approach using Support Vector Machines (SVM). Whenever the level of interest achieves an high value, the person is analyzed in more detail to detect the position and the motions of his/her arms as well as whether the person is shaking or nodding his/her head. This information is managed by a fuzzy system to detect a possible interest demand or the intention of the person to say yes or no using his/her head. At the end of the paper, some experiments are shown to validate the proposal.Finally, future work is addressed.