Low-barrier nanomagnets are attracting significant interest as building blocks (p-bits) of probabilistic computers with the promise of realizing compact random number generators fluctuating at sub-nanosecond time scales. Electrical characterization of such p-bits is challenging and slow as it would require building a full magnetic tunnel junction (MTJ) stack. In this work we propose quantum spin defects such as nitrogen vacancy centers (NV centers) as novel probes for rapidly characterizing magnetic p-bits. Through stochastic Landau-Lifshitz-Gilbert (LLG) equation simulations and analytical calculations, we show that NV relaxometry technique can be used to extract key parameters such as energy barrier and the so-called attempt time of a p-bit over a wide range of fluctuation frequencies. Moreover, thanks to the atomic size of the NV sensors and the weak magnetic fields they generate on the sample, this technique has ~10’s nm resolution and is noninvasive.