Doctors have an expression that they use: "dose dependent." This means that people will sometimes not have side effects to a certain medication or substance until they eat more than a certain amount. The side effects are "dependent" on the "dose."
The frequency and severity of the symptoms of lactose-intolerant seem to be dose-related. In general, the larger the amount of lactose consumed, the greater the risk of more frequent and severe symptoms.
Some can have 1/4 cup of milk on their cereal and not have problems but will experience symptoms if they drink a full glass of milk. Other people will have problems with as little as two tablespoons. Many people who cannot tolerate fresh milk can eat milk that has been cultured—like sour cream, yogurt and cheese — but, again, there are those who have problems with even these. For those who are very intolerant, even the lactose contained in some medications is enough to cause symptoms.
Here's a list of the lactose content in dairy (and other) foods that might help you.
Lactose is often found in foods where you might not expect it. For the most lactose intolerant this can be an issue and checking food labels for the following may help:
lactose, milk solids, nonfat milk solids, margarine, sweet cream, sour cream, buttermilk, whey, malted milk
Almost any food can contain lactose and checking the ingredient list is helpful especially with processed foods such as breads, candies, chocolate drink mixes, deli meats, cookies, canned soups. Dry cereals, frozen breaded fish and chicken, prepared and processed foods, salad dressing containing milk or cheese and artificial sweeteners can also contain lactose.
There are lactase supplements on the market (the most popular is the brand name Lactaid). For those that can tolerate a little but not a lot of lactose this can be a good strategy. By taking a pill containing the lactase with your meal that contains lactose it's easier to tolerate. There's a number of products on the market that also carry the Lactaid brand and many do well using such foods (although they do cost more).
Symptoms go a long way toward making a diagnosis. Stopping all lactose for a time is one place to begin. If your symptoms resolve, "rechallenge" yourself with a glass of milk or some ice cream. Sometimes it takes more than a single cup, however.
Your doctor can order lab tests and the easiest is the Hydrogen Breath Test. It measures any hydrogen in your breath before and after drinking a measured dose of lactose-containing liquid (usually milk). The test works because we don't normally exhale hydrogen. The bacteria in your colon (remember them?) do give off hydrogen when using the lactose and this is easily measured in your breath after consuming lactose.
Again, this is generally more of a problem in adulthood. Some children do have true lactose intolerance and it might be worth testing with their pediatrician to make sure that what appears to be lactose intolerance is not actually an allergy to milk and milk proteins, not a problem with milk sugars.
Even those who have lost the ability to make lactase don't generally lose it completely. They will often make a small amount - enough that they can tolerate small quantities of milk products.
There is an argument that those who avoid milk will end up being deficient in multiple nutrients including (but not limited to) calcium, B Vitamins, protein, Vitamin D, copper and zinc. This is plain silly and if you're eating a balanced diet there's no need for milk. The line of reasoning has been created by those in the dairy industry with the main threat being a decrease in calcium consumption.
If you are eating even a relatively healthy diet today you'll already be getting enough calcium. Starting the day with a bowl of one of the fortified cereals will give you as much as 1000 mg of calcium. Here's a list of the calcium content of other foods.