A rat will indicate that it is choking by gagging, drooling, pawing at its mouth, wiping its face on the bedding or crouching low while moving forwards or backwards. Some things that have caused choking are startling the rat while it is eating, picking up a rat while it is eating, and certain foods such as peanuts, peanut butter, mashed potatoes, really fresh bread, and cream cheese. Some rats just naturally eat too fast, and in their haste, something gets lodged in the throat. If the rat is still breathing, do nothing except to comfort and let the rat work it out on its own. Try the Heimlich Maneuver, or the Ratty Fling if the rat is not breathing and/or the rat is unconscious.
The ratty fling:
If the rat cannot breathe, AND you cannot get to a Vet, you could try decoupaging him (also called the "fling") - not to be tried if you are squeamish, or aren't exactly sure what to do, as you may cause more harm than good. Hold the rat at arms length above your head with index fingers under the chest and thumbs at the back, then bring the rat quickly and forcefully down to about waist level. You may have to do this a couple of times. You may then see him chewing, if the food is dislodged into his mouth. I would then suggest (if you are sure the obstruction is gone) leaving the rat somewhere peaceful and quiet for a short while to recover, as it is quite a frightening experience (for both you and the rat). Keep and eye and make sure all signs of choking have stopped.
http://health.ratzrus.co.uk/illnesses/choking.htm