  An interacting spin system is a great testbed for fundamental quantum physics
and applications in quantum sensing and quantum simulation. For these
investigations, detailed information of the interactions, e.g. the number of
spins and their interaction strengths, is often required. In this study, we
present the identification and characterization of a single nitrogen-vacancy
(NV) center coupled to two electron spins. In the experiment, we first identify
a well isolated single NV center, and characterize its spin decoherence time.
Then we perform NV-detected electron paramagnetic resonance (EPR) spectroscopy
to detect surrounding electron spins. From the analysis of the NV-EPR signal,
we determine the number of detected spins and their interaction strengths
precisely. Moreover, the spectral analysis indicates candidates of the detected
spins to be diamond surface spins. This study demonstrates a promising approach
for the identification and characterization of an interacting spin system for
realizing entangled sensing, using the electron spin as quantum reporters.
