We use the short-time inference scheme [Manikandan, Phys. Rev. Lett. 124, 120603 (2020)0031-900710.1103/PhysRevLett.124.120603], obtained within the framework of stochastic thermodynamics, to infer a lower bound to entropy production rate from flickering data generated by interference reflection microscopy of HeLa cells. We can clearly distinguish active cell membranes from their adenosine-triphosphate-depleted selves and even spatiotemporally resolve activity down to the scale of about 1 μm. Our estimate of activity is model independent.