There is no one specific symptom of dietary sensitivities in dogs, however common signs of either a food allergy or food intolerance can be:
You may find that your dog does not suffer from these problems all the time, more that they will come and go. Also it is possible for your dog to react to an ingredient that they have previously eaten with no problem.
NB. It is important to point out that these symptoms can be caused by other conditions so it is important to seek advice from your Veterinarian to rule these out.