Rule out fleas:
Fleas and flea allergy can cause similar symptoms to skin allergy including itching and infection, therefore it is important to ensure fleas are not the cause of your pet's allergy-like symptoms.
Eliminating fleas as a possible cause is fairly easy simply by ensuring your pet's flea treatments are done regularly.
Rule out other parasites:
If fleas have been ruled out, your vet can perform some tests to rule out other parasites such as demodex and other mites.
Rule out infection:
If one of the symptoms your pet is suffering from is a skin +/or ear infection, your vet will treat this with appropriate treatment.
If the infection recurs, then it is likely to be associated with an underlying allergy.
Rule out food allergy:
If the allergy symptoms remain after excluding or treating other causes so far, the next step is to exclude the possibility of food allergy.
The only way to prove food allergy is by doing a food trial, whereby you feed your pet a novel diet for a minimum of 8 weeks. Your vet may choose to run a test that can help identify which ingredients may be appropriate for a food trial. It is important to remember though that this does does not prove what food your pet is allergic to.
For more detailed information of food allergy, food allergy testing and diet trials:
Interpret history & clinical signs
After all previous steps have been taken, by looking at your pet's health history eg when did the symptoms start, how severe are they, when do they occur and location of symptoms etc, your vet will conclusively be able to diagnose allergy.
