Question: How can I tell the difference between lactose intolerance and a milk allergy?
Answer: Lactose intolerance is a problem with digesting lactose sugar in milk. So those symptoms generally are limited to some discomfort of the stomach -- some gas, some bloating, maybe some diarrhea -- there's a delayed onset of action, and the symptoms are inconsistent with eating milk or dairy products -- sometimes it happens, sometimes it doesn't.
In comparison, food allergic reactions usually are quick, they come on quickly. They're associated with other types of symptoms besides just of the stomach or the GI tract. So hives, difficulty breathing, allergic eye or nose symptoms are often common, typically occur.
They're also consistently reproducible. Every time you eat a certain amount of milk, that triggers the reaction that typically occurs, and it typically occurs quickly, almost always within an hour.