Normal breathing is steady, quiet, and happens twelve to twenty times per minute in adults. Agonal breathing is irregular, loud, and happens only two or three times per minute, sometimes less. Normal breathing delivers oxygen to the blood. Agonal gasping does not. Anyone showing agonal breathing while unresponsive is in cardiac arrest and needs CPR.
The simplest way to tell the difference is to look at the whole picture. A person breathing normally will be responsive, with a regular chest rise and fall and quiet airflow. A person with agonal breathing will be unresponsive, with infrequent gasps and often a snoring or gurgling sound. The chest may move briefly between gasps before stopping altogether.
If you cannot tell whether breathing is normal, treat it as not normal. The cost of starting CPR on someone who turns out to be fine is minor. The cost of not starting CPR on someone in cardiac arrest is fatal.