Recent progresses in the design of wearable RFID-tag antennas stimulate the idea of passive body-centric systems, wherein the required power to drive the wearable tags is directly scavenged from the interrogation signal emitted by the reader unit. While active body-centric links have been extensively investigated, the feasibility of passive systems is still questionable, due to the poor sensitivity of the tags and due to the modest reading distances. This paper describes a systematic measurement campaign involving low-profile wearable textile tags in the UHF RFID band. It was demonstrated that both on-body and off-body links are affordable, with a power budget fully compliant with the available technology and the safety standards. The experiments permitted identifying the most-efficient tag placements, and proposing some quantitative and general guidelines useful to characterize and design this kind of new system.