Many protocols in atomic physics and quantum information hinge on the ability to detect the presence of neutral atoms(1-4). Up to now, two avenues have been favoured: the direct detection of spontaneously emitted photons using high-quality optics(5-7), or the observation of changes in light transmission through cavity mirrors due to strong atom-photon coupling(8-11). Here, we present an approach that combines these two methods by detecting an atom in a driven cavity mode through the collection of spontaneous emission and forward scattering into an undriven, orthogonally polarized cavity mode. Moderate atom-cavity coupling enhances the signal, enabling the detection of multiple photons from the same atom. This real-time measurement can establish the presence of a single freely moving atom in less than 1 mu s with more than 99.7% confidence, using coincidence measurements to decrease the rate of false detections.