Near-surface nitrogen-vacancy (NV) centers have been created in diamond through low-energy implantation of 15N to sense electron spins that are external to the diamond. By performing double resonance experiments, we have verified the presence of g = 2 spins on a diamond crystal that was subjected to various surface treatments, including coating with a polymer film containing the free radical 2,2-diphenyl-1-picrylhydrazyl. Subsequent acid cleaning eliminated the spin signal without otherwise disrupting the NV center, providing strong evidence that the spins were at the surface. A clear correlation was observed between the strength of the external spin signal and the relaxation time T 2 for the six NV centers studied. We have developed a model that takes into account the finite correlation time of the fluctuating magnetic fields generated by the external spins, and used it to infer the signal strength and correlation time of the magnetic fields from these spins. This model also highlights the sensitivity advantage of active manipulation of the longitudinal spin component via double resonance over passive detection schemes that measure the transverse component of spin. © 2012 American Physical Society.