What tells you it is chemical. A cloud or mist with no fire. A strong or unusual smell — though many dangerous gases have none. Several people ill at once with the same symptoms: streaming eyes, coughing, dizziness, vomiting. Dead birds or animals. Hazard placards on a lorry, a tank or a rail wagon. Liquid spreading from a container.
Move. Uphill and upwind, and keep going until you are well clear. Gas follows the ground and the wind; distance is the only protection an untrained person has.
Do not go in. This is the instruction people ignore and it is why the casualty list grows. Rescue inside a contaminated area needs breathing apparatus and a suit, and a rescuer without them collapses next to the person they were reaching, doubling the work and halving the chance.
Remove clothing. Taking off outer clothing removes a large part of the contamination on a person. Cut it off rather than pulling it over the head, bag it, and keep it — the responders may need it. Do not scrub the skin; a gentle rinse with plenty of water where advised, and dry blotting where water is not.
No ignition sources. Many vapours are flammable. No lighters, no engines, and use a phone at a distance rather than beside the release.
Tell the emergency services what you can see from where you are. The placard number, the name on the tank, the colour of the cloud, how many people are affected and which way the wind is blowing. Do not go closer to read a label.
If you are told to shelter rather than move: inside, upstairs where the substance is heavier than air, windows and doors shut, ventilation and air conditioning off, one room with the fewest openings, and listen for the instruction to come out.
Source. World Health Organization, chemical incidents, https://www.who.int/health-topics/chemical-incidents ; United States Centers for Disease Control and Prevention, chemical emergencies, https://emergency.cdc.gov/chemical/index.asp
What this is not. Reading, not training. Follow the emergency services and the civil-protection authority where you are; where they differ from this page, they govern.