The cusp is a region in which the solar wind plasma can directly penetrate into the ionosphere. Most previous studies mainly focused on the low‐ and high‐latitude boundaries of the cusps, the identification of the dawnside and duskside boundaries of them appears to be questionable. The auroras are the ionospheric manifestations of solar‐terrestrial energy coupling. This paper presents a new approach to diagnose the duskside boundary of the cusp with the observation of a particular polar cap arc (PCA) located near the ∼1500 magnetic local time sector (15MLT‐PCA). Using measurements from Defense Meteorological Satellite Program satellite, we selected several 15MLT‐PCA examples where the satellite flew over different regions and analyze the particle characteristics. We identified the precipitating particles in dawnside and duskside regions next to 15MLT‐PCA are from the low latitude boundary layer/cusp and boundary plasma sheet/central plasma sheet respectively. The statistical result further confirms our findings. Therefore, we proposed that 15MLT‐PCA can be used as an identifiable optical signature of the duskside boundary of the cusp. This paper provides a valuable approach to identify the cusp boundaries, and the results are critical for understanding the generation mechanism of 15MLT‐PCA.