The methods used to diagnose milk allergies are very similar to those used in other food allergies. It can be relatively simple to diagnose a milk allergy when milk is already a suspected allergen trigger. 

The first step a physician is likely to take in the diagnosis of a milk allergy is to create a detailed medical history and dietary history of the patient. The most basic question a physician will ask is whether an individual has a consistent reaction each time milk and milk products are consumed. Other questions may focus on the frequency of reaction, timing of symptoms after eating a particular food and family history of allergies and other conditions (e.g., asthma, eczema).

The physician will also perform a physical examination to help identify or exclude medical problems that may be causing the patient's symptoms. To be sure that milk is the culprit, a physician will often administer one or more of the following tests:

• Skin test. This test involves scratching, pricking or injecting an individual’s skin with milk protein. In some cases, a patch soaked in milk protein is taped onto the patient’s skin instead. The tested area will react with redness or swelling to indicate an allergic response.   
  
  
• RAST (radioallergosorbent test). This type of blood test allows a laboratory to directly test a blood sample from an individual in an attempt to detect antibodies that correspond to a milk allergy.  
  
  
• Elimination diet. This test involves removing milk from an individual’s diet for several weeks to see if allergic reactions persist. If the reactions do indeed stop, it can be presumed that milk was the culprit.
  
  
• Oral food challenge. Different foods are placed within capsules to hide their identity. The patient consumes the capsules without knowing which capsule contains the suspected allergen and the physician looks for signs of an allergic reaction.