That *could* be a sign of a milk intolerance, but drinking too much cow's milk can also simply cause constipation. How much cow's milk was she drinking per day? Does she eat yogurt or cheese, or other dairy products? Does she drink water or juice? If she eats other dairy products, I'd just cut back on cow's milk and increase her water intake and make sure she's getting a good amount of fiber. Prunes/prune juice would also help as well.
A true allergy can be diagnosed through a blood RAST test and/or a skin prick test. The pedi can order the blood test, a pedi allergist would do the skin test. If it's an intolerance, that is diagnosed via symptoms, as there is no diagnostic test for an intolerance.