Bachelor group dissolution is the first visible signal. Through summer and early fall, mature bucks travel and feed in groups of two to five. Some time in mid-October those groups break, and you start seeing previously-grouped bucks alone. If you have summer trail-cam history, the day the bachelor group goes solo is a useful tag for pre-rut starting.
Fresh scrapes are the second. Bucks open scrape lines along ridge tops, saddles, and the downwind edges of doe bedding. Pre-rut scrapes are pawed bare to the dirt and the licking branch above is freshly worked. A scrape that's freshened overnight — leaves cleared, fresh urine — is pre-rut sign you can hunt next morning.
Third is buck movement during shooting light. Bucks that fed at midnight in early October start showing up at 5:30 PM as pre-rut hits. Cameras capturing daylight bucks where they were strictly nocturnal a week earlier means seeking is on.
Fourth is new bucks on camera. Pre-rut expands a buck's range significantly. The 4.5-year-old that's been on a neighbor's property all summer can show up on your camera as he scent-checks new ground. New cameras of a buck you've never seen is a rut tell.
Cross-reference these signals with the photoperiod-locked dates for your DMU on our state rut date pages — when ground sign aligns with the calendar, you're in pre-rut.