Look at the chest, rib cage, and flanks to see if there is a rise and fall, feel on your cheek or the back of your hand for breath coming from the nose, listen for breath sounds.  Check to see if any foreign bodies are lodged back in the throat.

Breathing:  If your rabbit is not breathing, pull the tongue out just a little, close the mouth and tilt their head back slightly to open the airway.  Administer 4-5 breaths mouth to the nose.  That is, close their mouth and gently breathe into their nose through your mouth. If squeamish about this, cover the nose with a light tissue, gauze or other flimsy material.  You want to breath out just enough to make the chest rise.  Larger rabbits will
need more breath; little rabbits will need much less.  Don't give too much or you will injure the lungs.  Do at a rate of once per second.

Circulation:  Check to see if their heart is beating.  Check for a heartbeat (pulse).  A pulse on a rabbit can usually be felt in the central artery of the ear.  A femoral pulse (inside of the rear leg, towards the top of the leg in the groin area) can sometimes be found but it is not as easily located as on a cat or dog.  If there is a pulse but no breathing, continue to perform mouth to nose resuscitation at the rate of 1 breath every second.   If there is no pulse, begin CPR.

For small animals like rabbits, the technique is different than it is for larger animals.  Place the rabbit flat on the ground.  Gently compress the heart between first finger and thumb at the rate of 70-90 times per minute.  Begin compressions at only ½-1 inch deep (depending on the size of the rabbit) and give one breath for every three compressions.  Do this gently as the rabbit's skeleton is fragile.  Check for pulse.