If you haven't been exposed and there are other people who have, you can be of the most use by helping to identify the material. Using binoculars or a spotting scope, try to find shipping placards on the outside of a truck or the NFPA fire diamond on fixed installations. Call 911 and describe the signage and situation to the dispatcher so that they can best prepare the emergency first responders. Always remember to keep your distance uphill, upwind, and upstream! DO NOT get close to the scene if you cannot identify the placard or sign!
US Department of Transportation Hazardous Materials Placard
These will only be seen on trucks, trains, or other vehicles that transport hazardous materials. If you can see this placard from safe distance, tell the 911 dispatcher the color of the placard, any symbols on it, and the 4 digit number in the white box (not all placards will have this). Vehicles generally only need to be labeled if they are carrying over 1000 pounds of a single material. If they are carrying a mixed load and each material is less than 1000 pounds, they are not required to have a placard.
National Fire Protection Agency Fire Diamond
You will find these on fixed installations, buildings, and facilities. If you can see this diamond from a safe distance away, tell the 911 dispatcher that it is an NFPA diamond and read off the colors with whatever number or letter is within it. Remember, there can still be hazardous materials inside of a building, but not enough to require the use of an NFPA fire diamond.