Opportunistic networks with cognitive management systems can improve the resource use in future wireless communication networks by forming local clusters that are temporary extensions of the infrastructure and governed by the operator. This paper presents a decision making system that selects the techniques for obtaining spectrum availability information in opportunistic networks. The proposed decision making system selects the most suitable technique(s) from cognitive control channels, databases, and spectrum sensing techniques. Moreover, a novel and simple rule-based expert system is developed to choose the spectrum sensing technique among energy detection, correlation-based detection, and waveform-based detection. The selection is made based on the required probability of detection, operational SNR, available time, and available a priori information. The developed rule-based decision making system is presented in the form of a decision tree to illustrate the dominating paths that influence the decisions. Situations where none of the considered spectrum sensing techniques can meet the given conditions are identified and new approaches are proposed including cooperative sensing and changing of the channel. Results are presented to verify the functioning of the proposed decision making system and to show the relative frequencies of the different selected sensing techniques. Significant improvements can be obtained when the decision making system is used compared to using a single sensing technique instead.