The fact that security is a critical problem when implementing mobile ad hoc networks (MANETs) is widely acknowledged. One of the different kinds of misbehavior a node may exhibit is selfishness. A selfish node wants to preserve its resources while using the services of others and consuming their resources. One way of preventing selfishness in a MANET is a detection and exclusion mechanism. In this paper, we focus on the detection and present different kinds of sensors that will find selfish nodes. First we present simulations that show the negative effects which selfish nodes cause in MANET. In the related work section we will analyze the detection mechanisms proposed by others. Our new detection mechanisms that we describe in this paper are called activity-based overhearing, iterative probing, and unambiguous probing. Simulation-based analysis of these mechanisms show that they are highly effective and can reliably detect a multitude of selfish behaviors.