Before a lunge, a worried dog may slow, freeze, turn away, lick their lips when there is no food, or stop responding as usual. These are clues, not a stopwatch or a diagnosis. No one sign means the same thing in every dog. Notice what changed from your dog's usual loose, comfortable movement.
Read the setting with the body
Look at where your dog is looking, how they move, and whether they can choose to turn away. A still dog is not necessarily a calm dog. If you see discomfort, create space from the person or dog ahead. Turn toward a safer route with a loose, secure hold on the leash, rather than forcing a greeting or demanding a sit.
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Learn the pattern when things are calm
Afterward, jot down the distance, the setting, and the first change you noticed. There is no guaranteed "three-second warning". Some dogs move from subtle signs to a reaction quickly, and some freeze. Keep people and other animals safely apart if there is bite risk. Ask a veterinarian about a new behavior change and a qualified behavior professional for a plan.