This work presents a technique to determine the minimum power necessary to turn on passive UHF RFID tags in quiet environments. For this purpose, the tags are tested in a small, low-cost GTEM cell that provides an ambient immune to electromagnetic radiation for the measurements. The technique can be used to determine the tags' reading rate as well. The procedure is inspired on the EPC global standards, especially regarding the number of reading attempts by the reader equipment used in the measuring setup. With the proposed technique, a fast determination of a tag's sensitivity is possible, in a very economical way. This enables one to optimize the deployment of RFID-based systems, taking into account the tag's sensitivities over the frequency band allowed by local spectrum coordinating agencies.