I suspect that I might be lactose-intolerant, but I don't know how to tell. Can I just see a doctor and get tested, or do I have to live with the assumption?
If there are any standardized tests, how is the procedure?
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This page from the NIH has a lot of relevant information about lactose intolerance.
There are several standard diagnostic tests for lactose intolerance, but your physician might ask you to try eliminating dairy from your diet before you receive any of those tests. If avoiding lactose alleviates your symptoms, you've potentially treated your primary lactose intolerance successfully. If you feel somewhat better but still experience classic GI problems (bloating, diarrhea), you might be lactose intolerant secondary to a different illness like Crohn's disease or celiac disease. Keep in mind, lactose intolerance is caused by the lack of an enzyme (lactase) in your body, so lactose can't be digested – a milk allergy is your body generating an immune response to certain milk proteins, but doesn't suggest any enzymatic deficit.
Usually, your physician will perform a hydrogen breath test to confirm a lactose intolerance diagnosis. When you're missing the lactase enzyme, you can't digest lactose (which means you can't absorb lactose, since it's "too big" pre-digestion – this and energy release are the goals of catabolism). This results in lactose sitting around post-consumption (there are no enzymes available to cleave it for transport), where it starts to ferment in your gut microbiota. Bacterial digestion like this makes degradation products like methane, carbon dioxide, and hydrogen (this is where the name "hydrogen breath test" comes from), and we can measure these gases as they rise to escape from your GI tract. A basic schematic:
Some people don't exhibit conventional responses to the hydrogen breath test, so a methane breath test or combination test might be more reliable. Alternatively, fecal pH tests can be used to show GI irregularities like lactose intolerance (malabsorption makes your stool acidic). More invasive techniques like blood glucose monitoring or even intestinal biopsy for disaccharidase quantitation can be employed, although the general patient population is well-suited for noninvasive tests like those that require only spent breath or stool and pH paper.