You may notice a strange smell in the air.
You may notice people suffering from some of the symptoms of an attack (coughing, choking, gasping for air, suffering from pain in the eyes, falling unconscious, convulsing, etc.).
You may start to suffer from some of these symptoms yourself.
It may seem to be getting dark.
You may notice a cloud of gas, hear an explosion, spot a crop duster where it shouldn't be, or something else unusual.
You will hear about the attack via radio, TV or warning sirens.
Depending on the nature of the attack, there's every chance that you will not notice anything unusual.
If the attack is with anthrax (which doesn't transmit from person to person), then you may notice unusual activity like those described in point 5 above.
However, if the attack involves spreading a contagious disease, then the terrorists will probably use a subtle approach (like putting it in a water supply or simply releasing it quietly among the general population). In this scenario, it will probably be days or weeks later - when more and more people start to suffer the initial symptoms of the attack - before you realize that it happened.
At this point it will be difficult to determine if you've been affected.
If you've any reason to suspect that you've been subject to a biological attack, follow the basic procedures described later and seek immediate medical attention.
Call your doctor or local hospital before visiting. This will prevent you spreading the disease to others - especially important medical staff. Follow whatever advice they give you on how to proceed.
Note: If you know that an attack has happened in your region, be on the look-out for flu-like symptoms. If you suffer these symptoms, call your doctor or local hospital immediately.