Pay attention to your heart rate. During a panic attack, your heart will race, beating as if it is going to come out of your chest.
Focus on your breathing. Often, you will take short, shallow breaths at quick intervals. This will result in your brain not getting enough oxygen. As a result, you will feel light headed. If you are feeling light headed and your heart is racing, you are probably having a panic attack.
Determine whether you are sweating. Panic attacks are often accompanied by profuse sweating. Even if you do not normally perspire very much, you will find beads of sweat on various parts of your body.
Notice how you are feeling. If you have an extreme feeling of dread or fear, even if you can't identify what the feeling relates to, you are probably having a panic attack. You will also likely feel as if the walls are closing in on you. This is another common experience that is closely related to the feeling of dread or fear that accompanies a panic attack.
Listen to what is going on around you. If you concentrate, you may find that you can't hear much of anything in your environment, or you may hear a high pitched ringing. This ringing in your ears happens because of the amount of blood and the force or speed it is traveling through your body. The blood is interfering with your ability to hear.
Relax. While it may feel like you are dying, a panic attack will not kill you. Employ some relaxation techniques and you should be able to control your panic attack. See resources below.