We present the design, implementation and evaluation of a location detection system built over a Radio Frequency network based on the IEEE 802.11 standard. Our system employs beacons to broadcast identifying packets from strategic positions within a building infrastructure in such a way that each resolvable position is covered by a unique collection of beacons; a user of such a system can thus determine his location by means of the beacon packets received. The locations from which beacons broadcast is determined from a formalization of the problem based on identifying codes over arbitrary graphs. We present experimental evidence that our location detecting system is practical and useful, and that it can achieve good accuracy even with a very small number of beacons.