Dark Knights can apparently sense each other. While its not directly stated that they can, there are two instances where two dark knights confirm this. The first being when Fray says that "There is a dark power growing within you." During the initial Dark Knight quest and when you talk to the Au Ra in the lvl 60 Dark Knight armor in the forgotten Knight. If you spoke to him before hand, he will tell you to leave but after the two initial DRK quests, he acknowledges that you wield the power of Darkness and to take care around the Temple knights, without you even telling him that you possessed such a power.
