You can use a peak flow meter, which is a device to check the lung capacity and breathing changes used for asthma patients. This device can help you in finding out whether you are about to have an asthma attack. If the device reading is in between 50 percent to 80 percent then the attack has already started. You should call for help or use your asthma inhaler right way if the device reads below 50 percent. Dial 911 if you are having problem walking or speaking due to the fact that you are unable to breathe, if your lips and fingers become blue or grey. Use your inhaler at the same time as you watch for assistance.