You'll know it. CPR is needed when the heart isn't pumping i.e. Cardiac arrest. In this situation the patient will remain unresponsive even if "fast and hard" chest compressions are being given. If the patient is not in cardiac arrest (sleeping, intoxicated, etc), they will respond in some way when compressions are begun as compressions are not pleasant. Look at it this way, if they wake up, your job is done.
It hurts. The chest compressions of CPR can break ribs, and would be very uncomfortable. Other aspects of advanced resuscitations like endotracheal intubation would also be uncomfortable. A "healthy person" would likely punch anybody who attempted CPR on them.
A punch in the face. If you try performing CPR on an awake, healthy person, they won't be happy. Cpr, when done properly, can break ribs and leave painful bruises. File this under stupid things teenage boys try on each other.
CPR ON HEALTHY PESON. You should not try it, it can be dagerous.