-May give you a warning look when you approach him while he has food
-May lower his head over the food
-May give you a warning growl when you approach his food
-His body may noticeably stiffen
Food Aggression is actually called Canine Possession Aggression (CPA) and can be a real challenge! It could be that the dog has had a time in his life that his food was taken away from him and he feels that he needs to guard it in order to eat.
There are different methods of dealing with the problem. I feel that a combination of different approaches works best.
There is the approach of showing your dog that you are the boss. I think that will work ONLY if your dog is being food aggressive JUST TO SHOW YOU that he can be. That is different than guarding food because he feels he has to protect it. Generally that is not the case. The dog is not just trying to prove that he can get away with it.
You will need to teach the dog that you are GIVING him food and not there to TAKE it and try to starve him. If the food aggressiveness is severe, you should not feed in the presence of other dogs or other people. Your dog may take the others as a threat and harm someone. Practice this alone with your dog.
Instead of putting his food in the dog dish, feed him from your hand. If you are fearful, do NOT try this. The dog needs to feel your sense of sharing and not a sense of fear. It will only confuse him if you are afraid. Share several pieces of dog food with him. Then put the food bowl on a countertop where the dog cannot reach it. Fill the bowl and set it down in the dogs normal eating place. While the dog is eating, gently toss a TREAT (something that smells great and will be tempting) near his eating spot. Do not invade his territory, but do try to toss the treat close him.
Do not try to remove his food bowl until he has finished eating and has left the area. Remove the food bowl when he is not present. Do not leave the food bowl on the floor as another dog or a human may get near it, and that may alert the guarding in the dog again.
This will take some time and you should not rush him. This was a learned behavior for him and it will take some patience to show him that he does not need to protect his food.