The Cognitive Radio is a solution proposed for the increasing demand of radio spectrum. Usually cognitive radios are adopted by the non-license wireless users, which have a certain degree of cognition in order to only access to a given frequency band when the band is sensed idle. However, these bands and their use must be assessed, to avoid interfering with licensed users (primary users). The way to assess band’s occupancy is by discerning between just noise or noise plus signal. In this paper, energy-based sensing (EBS) is considered through the use of a classical energy detector. The work proposes and describes an implementation of an energy detector using a software defined radio (SDR) testbed and, after computing the probability of detection and false alarm from a real set of samples obtained with the SDR devices, we successfully validate a theoretical model for the probabilities. EBS’ performance is validated for several points of operation, i.e. for different signal-to-noise-ratio (SNR) values. These findings may be useful for building an EBS detector that defines its own decision threshold in real time given the target probabilities, since the formal probabilities are successfully validated. Moreover, our contribution also includes a detailed description of the implemented blocks using GNU Radio’s open-source software development toolkit.