Owners can distinguish between play fighting and real aggression in cats by observing their body language and vocalizations. Play fighting is typically silent, with both cats taking turns being on top, and ends without any participant hiding or appearing stressed. In contrast, real aggression involves growling, hissing, or yowling, with ears pinned back and fur standing on end. Additionally, real fights tend to be more intense and one-sided, potentially leading to injuries. If these signs are evident, it indicates a shift from playful behavior to aggression.