Readability of passive RFID fundamentally depends on the tag empowering. The minimum required power, usually referred to as sensitivity, depends on the chip and antenna manufacturing. The sensitivity affects the readability but it is not the sole determinant of readability. The readability of RFID comprises a number of factors, particularly in the UHF band. This paper proposes a classification of tag readability degradation factors and measurement methods to quantify the factors. It is shown that tag performance change at the attachment may affect the readability. A tag probe, a power measurement antenna at the position of tag attachment, can monitor essential data to comprehend the situation and establish a countermeasure to improve the tag readability. The operating environment, involving reader antenna positions and standing waves result from the multipath fading, is another important factor for the readability. The characteristics of the environment could be also grasped by using the tag probe.