When playing a plucked string instrument, musicians are placed in a multisensory environment where all the cues perceived allow them to control and adapt their gestures to produce the desired sounds. With the emergence of Digital Musical Instruments (DMIs), the replication of haptic feedback resulting from the interaction between the musician and the instrument has gained in relevancy. A comprehensive understanding of the associated tactile cues is therefore essential to design haptic interactions with DMIs. To investigate the sensitivity of human touch when playing a plucked string instrument, we put in place an experiment in which participants had to discriminate guitar strings based on two of their key physical attributes, diameter and tension, only by plucking them. We found the Weber fraction associated with the 67% Just Noticeable Difference to be 1.53 for string diameter discrimination and 0.4 for string tension discrimination. These results suggest that, during a plucking action, human touch does not perceive as different the typical variations of these two parameters extrinsic to the string.