EPR, or ESR as it is also known, utilizies the resonance observed at an electron's own Larmor frequency when precessing about a static magnetic field. To gain a good understanding, I invite you to read two well-organized books on the subjects:
 Electron Paramagnetic Resonance: Elementary Theory and Practical Applications
John A. Weil and James R. Bolton, Wiley & Sons 2006
 Electron Paramagnetic Resonance: A Practitioners Toolkit
Marina Brustolon and Elio Giamello (editors), Wiley & Sons 2009
The spectra recorded by EPR machines is in essence the averaging over an almost incomprehensible amount of electrons. Therefore, these spectra may differ dramatically from that of a single electron, or even a single few. With our sensor, we can implement a direct method of probing the sample's electron spin, and also perform tests, which tell us whether this is a single spin or a larger spin bath. This method is called, again borrowing from the EPR community - Double Electron Electron Resonance, or DEER.
There have already been reports on the sensing of a single electron in diamond  due to a paramagnetic impurity, as well as the use of DEER to detect the spin-label from a single protein . Both show the feasibility of sensing single electron spins. However, there is no direct proof, so far, of truly sensing a single electron spin from an external sample (i.e., not diamond).
 Grinolds et al., Nat. Nanotech. 9, 279 (2014)
 Shi et al., Science 347, 1135 (2015)
One of the foci of our group, is to use a combined Atomic Force Microscope (AFM) and a Confocal Fluorescence Microscope (CFM) to perform DEER measurements (and their "offsprings"). The samples are typically spin-labels, which are the markers used by the traditional EPR community to infer structural information about molecules. We aim to do the same, but in the single-molecule limit.
Here is an exemplar of how a DEER measurement looks in our setup:
electron spin resonance of spin labels (top and middle) and dangling bond electron spins measured by the nitrogen-vacancy center in diamond using the double-electron-electron-resonance protocol