Neck swelling is the most visible signal. Driven by spiking testosterone, a mature buck's neck thickens dramatically through October and peaks in early November. Tarsal glands — the hocks — are scent-marked aggressively, often blackened with rub urine, and they put off a sour, ammonia-laden smell.
Behavior matches. A rutting buck walks with purpose between fixed points: scrape, ridge, pinch, doe trail, repeat. He doesn't browse. He doesn't post up to feed. He's running a route. That single observation — a buck moving with intent rather than feeding — is the cleanest signal that the rut is on.