Well, there are a few different options.
1.) Take her to a K9 allergy specialist for a consultation and allergy testing to find out what she is allergic to. This can be very expensive, so keep that in mind.
2.) Choose a food with a "novel" protein source. So, pick a high quality food that has a protein source she has never eaten before i.e. Duck & Potato or Venison etc. When you feed this, make sure she gets no other food or edible treats what so ever. Give at least 8 weeks for improvement. Sometimes, dogs are allergic to food preservatives and carb sources as well. It would be great if you could find a food where the protein source and grain source is something she has never had. In the mean time, I would make sure your current food doesn't contain wheat, corn or soy. Sometimes a simple switch to a high quality food without these ingredients gets rid of the problem.
What are you currently feeding?